Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Trading Arrangement: Trading Arrangement [Axis] Sale of stock, remaining amount available for issuance and sale Sale Of Stock, Remaining Stock Available For Issuance, Amount Sale Of Stock, Remaining Stock Available For Issuance, Amount Maximum High Maximum [Member] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Title of 12(b) Security Principal Amount Reported Value Measurement [Member] Subsequent Event Subsequent Event [Member] Fair Market Value Derivative Assets (Liabilities), at Fair Value, Net Derivative Instruments, Gain (Loss) [Table] Derivative Instruments, Gain (Loss) [Table] Adjustment to Compensation, Amount Adjustment to Compensation Amount Financial Instrument [Axis] Financial Instrument [Axis] Interest and dividend receivable Increase (Decrease) in Interest and Dividends Receivable Segment Reporting [Abstract] Payable for unsettled securities purchased Payable for unsettled securities purchased Payable, Investment, Purchase Schedule of Portfolio Investments by Levels in the Fair Value Hierarchy Fair Value, Assets Measured on Recurring Basis [Table Text Block] Notorious Topco, LLC Notorious Topco, LLC [Member] Notorious Topco, LLC Number of fee components Management And Service Fees, Number Of Components Management And Service Fees, Number Of Components Drawn Revolving Credit Facilities Drawn Revolving Credit Facilities [Member] Drawn Revolving Credit Facilities Derivative Instrument [Axis] Derivative Instrument [Axis] Non-controlled/Affiliated Investment, Affiliated Issuer, Noncontrolled [Member] Accrued but unpaid deferred financing costs Debt Issuance Costs Incurred but Not Yet Paid Debt Issuance Costs Incurred but Not Yet Paid Current income tax expense Current Income Tax Expense (Benefit) Conversion price (in dollars per share) Debt Instrument, Convertible, Conversion Price Promissory Note Promissory Note [Member] Promissory Note Entity Common Stock, Shares Outstanding Unfunded Debt Securities, Revolving Credit Facilities, Denominated Unfunded Debt Securities, Revolving Credit Facilities, Denominated [Member] Unfunded Debt Securities, Revolving Credit Facilities, Denominated Agreements Related Parties Related Party Transactions Disclosure [Text Block] Distributions related to non-controlling interest in NMNLC Payments to Noncontrolling Interests Investment Company, Nonconsolidated Subsidiary [Axis] Investment Company, Nonconsolidated Subsidiary [Axis] Old Repurchase Program Old Repurchase Program [Member] Old Repurchase Program Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Amortization of purchase discount Accretion (Amortization) of Discounts and Premiums, Investments Debt to equity ratio, maximum Debt Instrument, Covenant, Debt To Equity Ratio, Maximum Debt Instrument, Covenant, Debt To Equity Ratio, Maximum Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Number of reportable segments Number of Reportable Segments Schedule of Notional Amounts and Fair Values of the Company's Derivative Instruments Schedule of Derivative Instruments [Table Text Block] DCA Investment Holding, LLC DCA Investment Holding, LLC [Member] DCA Investment Holding, LLC Stock repurchase program, additional authorized amount Share Repurchase Program, Authorized, Amount, Increase Share Repurchase Program, Authorized, Amount, Increase NC Topco, LLC NC Topco, LLC [Member] NC Topco, LLC Entity Address, City or Town SBA Guaranteed Debentures, Issued September 23, 2015, Maturity Date September 1, 2025 SBA Guaranteed Debentures, Issued September 23, 2015, Maturity Date September 1, 2025 [Member] SBA Guaranteed Debentures, Issued September 23, 2015, Maturity Date September 1, 2025 Carrying Value Derivative, Fair Value, Net Market & Income Approach Valuation, Market And Income Approach [Member] Valuation, Market And Income Approach Statement of Financial Position [Abstract] Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Unearned income Investment Owned, Unearned Income Investment Owned, Unearned Income Level II Fair Value, Inputs, Level 2 [Member] Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [Text Block] Maximum secured debt ratio Debt Instrument, Covenant, Secured Debt Ratio, Maximum Debt Instrument, Covenant, Secured Debt Ratio, Maximum Net investment income (in dollars per share) Investment Company, Investment Income (Loss), Per Share, Basic Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Total coupon/interest rate Interest rate Interest rate Investment Interest Rate Other expenses Other Expenses United States UNITED STATES Fortis Solutions Group, LLC Fortis Solutions Group, LLC [Member] Fortis Solutions Group, LLC Dividend Income Dividend income Dividend Income, Operating Subsequent Event Type [Axis] Subsequent Event Type [Axis] Concentration Risk Type [Axis] Concentration Risk Type [Axis] Schedule of Long-Term Debt Instruments [Table] Schedule of Long-Term Debt Instruments [Table] Collateralized agreements or repurchase financings Repurchase Agreements, Collateral, Policy [Policy Text Block] Pre-Incentive Fee Net Investment Income Below Catch-Up Threshold Investment Management Agreement, Pre-Incentive Fee Net Investment Income Below Catch-Up Threshold [Member] Investment Management Agreement, Pre-Incentive Fee Net Investment Income Below Catch-Up Threshold Use of estimates Use of Estimates, Policy [Policy Text Block] Food & Beverage Food and Beverage Sector [Member] Related Party [Axis] Related and Nonrelated Parties [Axis] SBA-guaranteed debentures Guaranteed Debt Guaranteed Debt Cash received for purchase of undrawn portion of revolving credit or delayed draw facilities Proceeds from Purchase of Undrawn Portion of Investment, Operating Activity Proceeds from Purchase of Undrawn Portion of Investment, Operating Activity Software Software Software Sector [Member] Software Sector Document Period End Date New ACI Buyer LLC New ACI Buyer LLC [Member] New ACI Buyer LLC Preferred stock, shares outstanding (in shares) Preferred Stock, Shares Outstanding Face amount Debenture Amount Debt Instrument, Face Amount Earnings per share—basic Earnings Per Share, Basic [Abstract] Unfunded Debt Securities, Bridge Facilities Unfunded Debt Securities, Bridge Facilities [Member] Unfunded Debt Securities, Bridge Facilities Net decrease in shares outstanding (in shares) Stock Issued During Period, Shares, Period Increase (Decrease) Term of debt Debt Instrument, Term Treasury stock at cost, 13,399,539 and 5,213,541 shares held, respectively Treasury Stock, Common, Value Interest Rate Debt Instrument, Interest Rate, Effective Percentage Upfront fees percentage Guaranteed Debt, Upfront Fees Guaranteed Debt, Upfront Fees Numerator for diluted earnings (loss) per share Net Income (Loss) Attributable to Parent, Diluted Contributions related to non-controlling interest in NMNLC Proceeds from Noncontrolling Interests Second lien Debt Securities, Second Lien [Member] Debt Securities, Second Lien Entity Shell Company Bonterra LLC Bonterra LLC [Member] Bonterra LLC 2022 Convertible Notes 2022 Convertible Notes [Member] 2022 Convertible Notes Schedule of Investments Investment Holdings, Schedule of Investments [Table Text Block] Earnings (Loss) Per Share Earnings Per Share [Text Block] Earnings Per Share [Abstract] Document Fiscal Period Focus Capital gains based incentive fee payable Incentive Fee Payable, Capital-Gain-Based Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Capital transactions Investment Company, Capital Share Transaction, Increase (Decrease) [Abstract] Total liabilities Liabilities Trading Symbol Discount rate Measurement Input, Discount Rate [Member] Increases in dividend in excess (in dollars per share) Debt Instrument, Convertible, Dividends In Excess, Increase Debt Instrument, Convertible, Dividends In Excess, Increase Borrowing capacity, fourth percentage Line Of Credit Facility, Borrowing Capacity, Fourth Percentage Line Of Credit Facility, Borrowing Capacity, Fourth Percentage Company Selected Measure Name Company Selected Measure Name Sale of Stock [Domain] Sale of Stock [Domain] Derivative fixed interest rate Derivative, Fixed Interest Rate Management fee waived Less: management fee waiver Investment Company, Management Fee, Contractual Fee Waived TigerConnect, Inc. TigerConnect, Inc. [Member] TigerConnect, Inc. Earnings per share—diluted Earnings Per Share, Diluted [Abstract] DOCS, MSO, LLC DOCS, MSO, LLC [Member] DOCS, MSO, LLC Investment Funds (includes investments in joint ventures) Investment Funds (includes investments in joint ventures) Investment Funds, Including Investments In Joint Ventures [Member] Investment Funds, Including Investments In Joint Ventures Credit Facility [Domain] Credit Facility [Domain] Cash flows from operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] SBIC II New Mountain Finance SBIC II, L.P. [Member] New Mountain Finance SBIC II, L.P. Distributions paid per share (in dollars per share) Common Stock, Dividends, Per Share, Cash Paid Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] Line of Credit Facility [Table] Line of Credit Facility [Table] Transfers out of Level III Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 Maximum purchase amount Debt Instrument, Repurchase Face Amount, Maximum Debt Instrument, Repurchase Face Amount, Maximum SBA Guaranteed Debentures, Issued September 19, 2018, Maturity Date September 1, 2028 SBA Guaranteed Debentures, Issued September 19, 2018, Maturity Date September 1, 2028 [Member] SBA Guaranteed Debentures, Issued September 19, 2018, Maturity Date September 1, 2028 Interest and dividend receivable Dividend and Interest Receivable Financial Support to Nonconsolidated Legal Entity [Axis] Financial Support to Nonconsolidated Legal Entity [Axis] Available borrowing capacity Line of Credit Facility, Current Borrowing Capacity Purchase amount per $1,000 principal amount Debt Instrument, Repurchase Amount, Per USD1000 Of Principal Debt Instrument, Repurchase Amount, Per USD1000 Of Principal Investments threshold period past due for nonaccrual status Investment Owned, Nonaccrual, Threshold Period Past Due Investment Owned, Nonaccrual, Threshold Period Past Due Schedule of Changes in Fair Value of Level III Portfolio Investments Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Deferred offering costs Deferred Charges, Policy [Policy Text Block] Other general and administrative expenses Other General and Administrative Expense Investment Company, Financial Highlights [Roll Forward] Investment Company, Financial Highlights [Roll Forward] Award Timing Predetermined Award Timing Predetermined [Flag] Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] Level I Fair Value, Inputs, Level 1 [Member] Nelipak Holding Company Nelipak Holding Company [Member] Nelipak Holding Company Conversion rate Debt Instrument, Convertible, Conversion Ratio Total square feet Area of Real Estate Property Summary of Investment Holdings [Line Items] Summary of Investment Holdings [Line Items] Financial Services & Technology Financial Services And Technology Sector [Member] Financial Services And Technology Sector AWP Group Holdings, Inc. AWP Group Holdings, Inc. [Member] AWP Group Holdings, Inc. Income Approach Valuation, Income Approach [Member] Net expenses Investment Company, Expense, after Reduction of Fee Waiver and Reimbursement Preferred stock, par value (in dollars per share) Preferred Stock, Par or Stated Value Per Share Borrowings Long-Term Debt, Fair Value Number of portfolio companies Investment Company, Number Of Portfolio Companies Investment Company, Number Of Portfolio Companies Concentration Risk Type [Domain] Concentration Risk Type [Domain] Holdings Credit Facility Holdings Credit Facility [Member] Holdings Credit Facility Fair Value Disclosures [Abstract] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Credit facility Credit facility outstanding balance Long-Term Line of Credit Distributions paid Payments of Ordinary Dividends, Common Stock New ACI Holdings LLC New ACI Holdings LLC [Member] New ACI Holdings LLC Expiration Date Trading Arrangement Expiration Date Net decrease in cash and cash equivalents Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Excluding Exchange Rate Effect, Including Discontinued Operation Unfunded Debt Securities, Revolving Credit Facilities Unfunded Debt Securities, Revolving Credit Facilities [Member] Unfunded Debt Securities, Revolving Credit Facilities Hedging Relationship [Axis] Hedging Relationship [Axis] Derivative asset, fair value, offset against collateral, net Derivative Asset, Subject to Master Netting Arrangement, after Offset and Deduction Total net assets New Mountain Finance Corporation net assets at the beginning of the period New Mountain Finance Corporation net assets at the end of the period Equity, Including Portion Attributable to Noncontrolling Interest Subordinated Subordinated Debt [Member] Commitment fee percentage Guaranteed Debt, Commitment Fee Guaranteed Debt, Commitment Fee Affiliates Affiliated Entity [Member] Unsecured Notes Unsecured notes outstanding balance Unsecured Debt Meta Buyer LLC Meta Buyer LLC [Member] Meta Buyer LLC Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] Net proceeds from shares sold Proceeds from Issuance of Common Stock Net investment income Net investment income Investment Income, Operating, after Expense and Tax TMK Hawk Parent, Corp. TMK Hawk Parent, Corp. [Member] TMK Hawk Parent, Corp. Ultimus Group Midco, LLC Ultimus Group Midco, LLC [Member] Ultimus Group Midco, LLC Related Party Transaction [Axis] Related Party Transaction [Axis] Asset Derivative Asset [Abstract] Interest rate swaps Interest Rate Swap [Member] Pre-Incentive Fee Net Investment Income Exceeds Catch-Up Threshold Investment Management Agreement, Pre-Incentive Fee Net Investment Income Exceeds Catch-Up Threshold [Member] Investment Management Agreement, Pre-Incentive Fee Net Investment Income Exceeds Catch-Up Threshold Low Voltage Holdings Inc. Low Voltage Holdings Inc. [Member] Low Voltage Holdings Inc. Deferred financing costs (net of accumulated amortization of $50,994 and $45,302, respectively) Deferred financing costs (net of accumulated amortization) Debt Issuance Costs, Net Average weighted shares outstanding for the period (in shares) Weighted Average Number Of Shares Outstanding Weighted Average Number Of Shares Outstanding Shares sold (in shares) Investment Owned, Shares Sold Investment Owned, Shares Sold Cardinal Parent, Inc. Cardinal Parent, Inc. [Member] Cardinal Parent, Inc. Other Other Valuation Technique [Member] Other Valuation Technique PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount SBA Guaranteed Debentures, Issued September 25, 2019, Maturity Date September 1, 2029 SBA Guaranteed Debentures, Issued September 25, 2019, Maturity Date September 1, 2029 [Member] SBA Guaranteed Debentures, Issued September 25, 2019, Maturity Date September 1, 2029 Stated interest rate Debt Instrument, Interest Rate, Stated Percentage Total net assets of New Mountain Finance Corporation Equity, Attributable to Parent Interest expense recognized on derivative Interest Expense Recognized on Derivative Interest Expense Recognized on Derivative Investment Company, Nonconsolidated Subsidiary [Table] Investment Company, Nonconsolidated Subsidiary [Table] Securities purchased under collateralized agreements to resell Net change in unrealized depreciation of securities purchased under collateralized agreements to resell Net change in unrealized depreciation of securities purchased under collateralized agreements to resell Unrealized Gain (Loss), Securities Repurchased Under Agreements To Resell Unrealized Gain (Loss), Securities Repurchased Under Agreements To Resell Incentive fee payable Incentive Fee Payable Accrued but unpaid offering costs Stock Issuance Costs Incurred but Not Yet Paid Stock Issuance Costs Incurred but Not Yet Paid GC Waves Holdings, Inc. GC Waves Holdings, Inc. [Member] GC Waves Holdings, Inc. Assets Assets [Abstract] EBITDA multiple Measurement Input, EBITDA Multiple [Member] Receivable from broker Receivable from Broker-Dealer and Clearing Organization Professional fees Professional Fees Insider Trading Policies and Procedures [Line Items] Investment, Issuer Affiliation [Domain] Investment, Issuer Affiliation [Domain] Deferred income tax provision Deferred Income Tax Expense (Benefit) Issuance discount percentage Guaranteed Debt, Issuance Discount Guaranteed Debt, Issuance Discount Number of operating segments Number of Operating Segments Entity Central Index Key Investment Company, Changes in Net Assets [Table] Investment Company, Change in Net Assets [Table] Equity [Abstract] Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Schedule of Investments [Line Items] Schedule of Investments [Line Items] Net realized and unrealized losses (in dollars per share) Investment Company, Gain (Loss) on Investment, Per Share Designated as Hedging Instrument Designated as Hedging Instrument [Member] Schedule of Current Income Tax Expense, Deferred Income Tax Provision and Total Income Tax Provision Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Net realized and unrealized losses Gain (Loss), Investment and Derivative, Operating, and Foreign Currency Transaction, after Tax Investments Net change in unrealized depreciation of investments and New Mountain Net Lease Corporation Net change in unrealized appreciation (depreciation) of investments Debt and Equity Securities, Unrealized Gain (Loss) Basis spread on variable rate Debt Instrument, Basis Spread on Variable Rate GS Acquisitionco, Inc. GS Acquisitionco, Inc. [Member] GS Acquisitionco, Inc. All Individuals All Individuals [Member] YLG Holdings, Inc. YLG Holdings, Inc. [Member] YLG Holdings, Inc. Investment, Issuer Name [Domain] Investment, Issuer Name [Domain] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Investments Net realized losses (gains) on investments Net realized losses on investments Debt and Equity Securities, Realized Gain (Loss) SBA Guaranteed Debentures, Issued September 20, 2017, Maturity Date September 1, 2027 SBA Guaranteed Debentures, Issued September 20, 2017, Maturity Date September 1, 2027 [Member] SBA Guaranteed Debentures, Issued September 20, 2017, Maturity Date September 1, 2027 Statement [Line Items] Statement [Line Items] Healthspan Buyer, LLC Healthspan Buyer, LLC [Member] Healthspan Buyer, LLC Total assets Assets Interest and other financing expenses Interest and other financing expenses Interest Expense, Operating Regulation Regulatory Capital Requirements For Investment Companies [Text Block] Regulatory Capital Requirements For Investment Companies Industry Sector [Axis] Industry Sector [Axis] Convertible Notes Convertible Debt [Member] Realized Capital Gains Investment Management Agreement , Realized Capital Gains [Member] Investment Management Agreement , Realized Capital Gains Community Management Holdings MidCo 2, LLC Community Management Holdings MidCo 2, LLC [Member] Community Management Holdings MidCo 2, LLC PEO PEO [Member] Beginning of the period (in shares) End of the period (in shares) Shares, Outstanding Net realized (losses) gains: Net Realized Gains (Losses) [Abstract] Net Realized Gains (Losses) Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount NM NL Holdings LP / NM GP Holdco LLC NM NL Holdings LP / NM GP Holdco LLC [Member] NM NL Holdings LP / NM GP Holdco LLC Investment Company, Changes in Net Assets [Line Items] Investment Company, Change in Net Assets [Line Items] Revolving Credit Facility Revolving Credit Facility [Member] Fair value Fair Value Measurement, Policy [Policy Text Block] Total net (decrease) increase (in dollars per share) Investment Company, Investment Income (Loss) from Operations, Per Share Number of shares outstanding (in shares) Common stock, outstanding (in shares) Shares outstanding at end of period (in shares) Common Stock, Shares, Outstanding Award Timing Disclosures [Line Items] Diluted earnings (loss) per share (in dollars per share) Diluted earnings (loss) per share (in dollars per share) Earnings Per Share, Diluted Incentive fee Incentive Fee Expense Litigation Case [Domain] Litigation Case [Domain] Interest income (excluding Payment-in-kind ("PIK") interest income) Interest Income, Operating, Paid in Cash Proceeds from credit facility Proceeds from Long-Term Lines of Credit Cronos Crimson Holdings, Inc. (f/k/a NMC Crimson Holdings, Inc.) Cronos Crimson Holdings, Inc. (f/k/a NMC Crimson Holdings, Inc.) [Member] Cronos Crimson Holdings, Inc. (f/k/a NMC Crimson Holdings, Inc.) FS WhiteWater Borrower, LLC FS WhiteWater Borrower, LLC [Member] FS WhiteWater Borrower, LLC Regulatory capital Small Business Association Regulation, Regulatory Capital, Actual Small Business Association Regulation, Regulatory Capital, Actual Net increase (decrease) in members' capital Increase (Decrease) In Member's Capital Increase (Decrease) In Member's Capital Industry Type Industry Concentration Risk [Member] Industry Concentration Risk Repayment of SBA-guaranteed debentures Total SBA-guaranteed debentures repaid Repayments of Other Long-Term Debt Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.) Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.) [Member] Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.) Total return based on market value Investment Company, Total Return, Market Value Number of collateralized agreements Number Of Collateralized Agreements Number Of Collateralized Agreements Repurchase Program Repurchase Program [Member] Repurchase Program Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Unfunded Debt Securities, Delayed Draws Or Other Future Funding Commitments Unfunded Debt Securities, Delayed Draws Or Other Future Funding Commitments [Member] Unfunded Debt Securities, Delayed Draws Or Other Future Funding Commitments PPV Intermediate Holdings, LLC PPV Intermediate Holdings, LLC [Member] PPV Intermediate Holdings, LLC Interest payable Increase (Decrease) in Interest Payable, Net Schedule of Summarized Financial Information for Nonconsolidated Entities Investment Company, Nonconsolidated Subsidiary, Summarized Financial Information [Table Text Block] Investment, Issuer Name [Axis] Investment, Issuer Name [Axis] Black Elk Trustee Black Elk Trustee [Member] Black Elk Trustee Net Unrealized Appreciation (Depreciation) Retained Earnings, Net Unrealized Appreciation (Depreciation) [Member] Retained Earnings, Net Unrealized Appreciation (Depreciation) Shares issued in debt conversion (in shares) Debt Conversion, Converted Instrument, Shares Issued Minimum asset coverage ratio Asset Coverage, Required Ratio, Minimum Asset Coverage, Required Ratio, Minimum New Mountain Net Lease Corporation New Mountain Net Lease Corporation [Member] New Mountain Net Lease Corporation Interest Rate Type Interest Rate Type Risk [Member] Interest Rate Type Risk Sale of Stock [Axis] Sale of Stock [Axis] (Losses) gains recognized Derivative, Gain (Loss) on Derivative, Net Members' capital Members' Equity Related Party Transaction [Domain] Related Party Transaction [Domain] Line Of Credit, Class A Advance Line Of Credit, Class A Advance [Member] Line Of Credit, Class A Advance New Benevis Holdco, Inc. New Benevis Holdco, Inc. [Member] New Benevis Holdco, Inc. Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] ICAT Intermediate Holdings, LLC ICAT Intermediate Holdings, LLC [Member] ICAT Intermediate Holdings, LLC Investments at fair value Fair value, beginning balance Fair value, ending balance Fair Value Investment Owned, Fair Value Equity Equity Securities [Member] Administrative expenses Administrative Fees Expense Common stock, issued (in shares) Common Stock, Shares, Issued Total net management fee Investment Company, Management Fee Expense, After Reduction Of Fee Waiver Investment Company, Management Fee Expense, After Reduction Of Fee Waiver Beginning balance Ending balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Sierra Hamilton Holdings Corporation Sierra Hamilton Holdings Corporation [Member] Sierra Hamilton Holdings Corporation Pre-Incentive Fee Net Investment Income Investment Management Agreement, Pre-Incentive Fee Net Investment Income [Member] Investment Management Agreement, Pre-Incentive Fee Net Investment Income Borrowings allowable per dollar of equity Debt Outstanding Allowable Per Dollar Of Equity Debt Outstanding Allowable Per Dollar Of Equity Repurchase of common stock (in dollars per share) Investment Company, Share Repurchase, Per Share Power Grid Holdings, Inc. Power Grid Holdings, Inc. [Member] Power Grid Holdings, Inc. Receivable from unsettled securities sold Increase (Decrease) in Receivables under Repurchase Agreements Deferred tax liability Deferred Income Tax Liabilities, Net Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] Termination Date Trading Arrangement Termination Date Other Commitments [Axis] Other Commitments [Axis] SBA Guaranteed Debentures, Issued March 21, 2018, Maturity Date March 1, 2028 SBA Guaranteed Debentures, Issued March 21, 2018, Maturity Date March 1, 2028 [Member] SBA Guaranteed Debentures, Issued March 21, 2018, Maturity Date March 1, 2028 Subsequent Events [Abstract] Aggregate principal amount converted Debt Conversion, Original Debt, Amount Fair Value, Recurring and Nonrecurring [Table] Fair Value, Recurring and Nonrecurring [Table] Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] After Reinvestment Period After Reinvestment Period [Member] After Reinvestment Period Fair Value as of Grant Date Award Grant Date Fair Value Repayment of 2022 Convertible Notes Repayments of Convertible Debt Redemption as a percentage of principal Debt Instrument, Redemption Price, Percentage Total net incentive fee Investment Company, Inventive Fee Expense, After Reduction Of Fee Waiver Investment Company, Inventive Fee Expense, After Reduction Of Fee Waiver OEConnection LLC OEConnection LLC [Member] OEConnection LLC Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Share repurchase program, remaining authorized, amount Share Repurchase Program, Remaining Authorized, Amount Arrangement Duration Trading Arrangement Duration Entity Address, Postal Zip Code Entity Registrant Name Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Schedule of Principal Amounts and Fair Values of the Company’s Borrowings Fair Value Disclosure of Asset and Liability Not Measured at Fair Value [Table Text Block] Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount PIK interest income Interest Income, Operating, Paid in Kind SkyKnight II SkyKnight Income II, LLC [Member] SkyKnight Income II, LLC (Decrease) increase in operating liabilities: Increase (Decrease) in Operating Liabilities [Abstract] Borrowing capacity, third percentage Line Of Credit Facility, Borrowing Capacity, Third Percentage Line Of Credit Facility, Borrowing Capacity, Third Percentage Reimbursements to administrators as a percentage of gross assets Investment Company, Fee Reimbursement To Gross Assets Investment Company, Fee Reimbursement To Gross Assets All Award Types Award Type [Domain] Fair Value Fair Value Disclosures [Text Block] Revenue multiple Measurement Input, Revenue Multiple [Member] Level III Fair Value, Inputs, Level 3 [Member] Non-Extending Lenders Non-Extending Lenders [Member] Non-Extending Lenders Shares repurchased under the repurchase programs (in shares) Shares repurchased under repurchase program (in shares) Repurchases of common stock under the Repurchase Program (in shares) Treasury Stock, Shares, Acquired Geographical [Axis] Geographical [Axis] October 2031 Tranche B Notes October 2031 Tranche B Notes [Member] October 2031 Tranche B Notes Incentive fee waived Less: incentive fee waiver Investment Company, Incentive Fee, Contractual Fee Waived Investment, Issuer Affiliation [Axis] Investment, Issuer Affiliation [Axis] Other assets Other Assets Non-cash operating activities: Non Cash Operating And Investing Activities [Abstract] Non Cash Operating And Investing Activities Legal Entity [Axis] Interest expense Interest Expense, Debt UniTek Global Services, Inc. UniTek Global Services, Inc. [Member] UniTek Global Services, Inc. DT1 Midco Corp. DT1 Midco Corp [Member] DT1 Midco Corp Repurchase of shares under the repurchase programs Common stock repurchased Repurchases of common stock under the Repurchase Program Treasury Stock, Value, Acquired, Cost Method Cash and cash equivalents at the beginning of the period Cash and cash equivalents at the end of the period Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Unfunded Investments Unfunded Securities [Member] Unfunded Securities Business Products Business Products Business Products Sector [Member] Business Products Sector Non-NEOs Non-NEOs [Member] New Mountain Capital New Mountain Capital [Member] New Mountain Capital Accumulated Net Investment Income Retained Earnings, Accumulated Net Investment Income (Loss) [Member] Retained Earnings, Accumulated Net Investment Income (Loss) Atlas AU Bidco Pty Ltd Atlas AU Bidco Pty Ltd [Member] Atlas AU Bidco Pty Ltd Long-Term Debt, Type [Domain] Long-Term Debt, Type [Domain] Repurchase of shares under the repurchase programs Payments for Repurchase of Common Stock Statistical Measurement [Axis] Statistical Measurement [Axis] Bullhorn, Inc. Bullhorn, Inc. [Member] Bullhorn, Inc. Base management fee percentage Management and Service Fees, Base Rate Schedule of Effect of Hedging Derivative Instruments Derivative Instruments, Gain (Loss) [Table Text Block] Structured Finance Obligations Structured Finance Obligations [Member] Structured Finance Obligations Award Type Award Type [Axis] PDI TA Holdings, Inc. PDI TA Holdings, Inc. [Member] PDI TA Holdings, Inc. Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Investments in Majority-owned Subsidiaries Investment, Affiliated Issuer, Controlled, Majority-Owned [Member] Net change in unrealized (depreciation) appreciation of investments, foreign currency and New Mountain Net Lease Corporation Net Change In Unrealized Appreciation (Depreciation) Unrealized Gain (Loss), Investment, Derivative, and Foreign Currency Transaction Price Change, Operating, before Tax Accumulated overdistributed earnings Retained Earnings (Accumulated Deficit) Payable for unsettled securities purchased Increase (Decrease) in Payable for Investment Purchased Summary of Investment Holdings [Table] Investment Holdings [Table] Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Common stock, par value $0.01 per share Common Stock Common Stock [Member] October 2027 Notes October 2027 Notes [Member] October 2027 Notes SBA Annual Charge Debt Instrument, Annual Charge, Percentage Debt Instrument, Annual Charge, Percentage Derivative Contract [Domain] Derivative Contract [Domain] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Incentive fee payable Increase (Decrease) in Incentive Fee Payable Eclipse Buyer Inc. Eclipse Buyer Inc. [Member] Eclipse Buyer Inc. 8.250% Notes due 2028 November 2028 Notes November 2028 Notes [Member] November 2028 Notes Schedule of Information Related to Convertible Notes Convertible Debt [Table Text Block] Total liabilities and net assets Liabilities and Equity Employee Stock Option Share-Based Payment Arrangement, Option [Member] Purchases, including capitalized PIK and revolver fundings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Business Services Business Services Business Services Sector [Member] Business Services Sector Securities Financing Transaction [Table] Security Financing Transaction [Table] Non-controlling interest in New Mountain Net Lease Corporation Equity, Attributable to Noncontrolling Interest Unearned interest income Investment Owned, Unearned Interest Income Investment Owned, Unearned Interest Income Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Asset Class [Axis] Asset Class [Axis] Related Party [Domain] Related and Nonrelated Parties [Domain] Total net decrease in net assets resulting from capital transactions Investment Company, Capital Share Transaction, Increase (Decrease) Tabular List, Table Tabular List [Table Text Block] Paid in capital in excess of par Additional Paid in Capital SBA Guaranteed Debentures, Issued September 23, 2020, Maturity Date September 1, 2030 SBA Guaranteed Debentures, Issued September 23, 2020, Maturity Date September 1, 2030 [Member] SBA Guaranteed Debentures, Issued September 23, 2020, Maturity Date September 1, 2030 Distribution & Logistics Distribution & Logistics Transportation Sector [Member] Adjustment for dilutive effect of 2022 Convertible Notes (in shares) Incremental Common Shares Attributable to Dilutive Effect of Conversion of Debt Securities Name Awards Close in Time to MNPI Disclosures, Individual Name Investment, Identifier [Axis] Investment, Identifier [Axis] Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested [Member] Total net derivatives Total net derivatives Derivative Asset (Liability), Net, Subject To Master Netting Arrangement, Before Offset Derivative Asset (Liability), Net, Subject To Master Netting Arrangement, Before Offset Sale of stock, price per share (in dollars per share) Sale of Stock, Price Per Share Associations, Inc. Associations, Inc. [Member] Associations, Inc. During Reinvestment Period During Reinvestment Period [Member] During Reinvestment Period Property, Plant, and Equipment [Table] Property, Plant, and Equipment [Table] Preferred stock, par value $0.01 per share, 2,000,000 shares authorized, none issued Preferred Stock, Value, Issued Adjustment to Compensation: Adjustment to Compensation [Axis] Receivable from broker Increase (Decrease) in Receivable from Broker-Dealer and Clearing Organization Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Cash interest paid Interest Paid, Excluding Capitalized Interest, Operating Activity Hedging Designation [Axis] Hedging Designation [Axis] Borrowing capacity, first percentage Line Of Credit Facility, Borrowing Capacity, First Percentage Line Of Credit Facility, Borrowing Capacity, First Percentage Credit Facility [Axis] Credit Facility [Axis] Name Measure Name Pay vs Performance Disclosure [Line Items] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Eclipse Topco Holdings, Inc. (fka Transcendia Holdings, Inc.) Eclipse Topco Holdings, Inc. (fka Transcendia Holdings, Inc.) [Member] Eclipse Topco Holdings, Inc. (fka Transcendia Holdings, Inc.) Debt Instrument [Line Items] Debt Instrument [Line Items] Paid in Capital in Excess of Par Additional Paid-in Capital [Member] AmeriVet Partners Management, Inc. AmeriVet Partners Management, Inc. [Member] AmeriVet Partners Management, Inc. Funded Investments Funded Securities [Member] Funded Securities Name Trading Arrangement, Individual Name DCA Buyer LLC DCA Buyer LLC [Member] DCA Buyer LLC Cash and cash equivalents Cash and Cash Equivalent Company Selected Measure Amount Company Selected Measure Amount Jawbreaker Parent, Inc. Jawbreaker Parent, Inc. [Member] Jawbreaker Parent, Inc. Repurchase amount Investment Owned, Balance, Repurchase Amount Investment Owned, Balance, Repurchase Amount Non-PEO NEO Non-PEO NEO [Member] Net expense recognized on fair value hedge Gain (Loss) on Fair Value Hedges Recognized in Earnings Income tax expense Investment Income, Operating, Tax Expense (Benefit) Award Timing Method Award Timing Method [Text Block] Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] eResearchTechnology, Inc. eResearchTechnology, Inc. [Member] eResearchTechnology, Inc. Net increase (decrease) in net assets resulting from operations related to New Mountain Finance Corporation Numerator for basic earnings (loss) per share Numerator for increase (decrease) in net assets per share Net Income (Loss) Attributable to Parent Long-Term Debt, Type [Axis] Long-Term Debt, Type [Axis] Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Total Investments [Member] Restatement Determination Date: Restatement Determination Date [Axis] Settlement amount awarded to other party Litigation Settlement, Amount Awarded to Other Party October 2028 Notes, October 2031 Tranche B Notes, and October 2031 Tranche C Notes October 2028 Notes, October 2031 Tranche B Notes, And October 2031 Tranche C Notes [Member] October 2028 Notes, October 2031 Tranche B Notes, And October 2031 Tranche C Notes Interest and other financing expenses Interest Expense, Policy [Policy Text Block] ACI Group Holdings, Inc. ACI Group Holdings, Inc. [Member] ACI Group Holdings, Inc. Investment income Revenues [Abstract] Einstein Parent, Inc. Einstein Parent, Inc. [Member] Einstein Parent, Inc. SBA Guaranteed Debentures, Issued March 25, 2020, Maturity Date March, 1 2030 SBA Guaranteed Debentures, Issued March 25, 2020, Maturity Date March, 1 2030 Two [Member] SBA Guaranteed Debentures, Issued March 25, 2020, Maturity Date March, 1 2030 Two Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Adjustment for interest on 2022 Convertible Notes and incentive fees, net Dilutive Securities, Effect on Basic Earnings Per Share, Dilutive Convertible Securities Financial Support to Nonconsolidated Legal Entity [Domain] Financial Support to Nonconsolidated Legal Entity [Domain] Borrowings Long-Term Debt, Unclassified [Abstract] Statistical Measurement [Domain] Statistical Measurement [Domain] NMFC Credit Facility NMFC Credit Facility [Member] NMFC Credit Facility Document Fiscal Year Focus Common stock, par value $0.01 per share, 200,000,000 shares authorized, 107,851,929 and 107,851,929 shares issued, respectively, and 94,452,390 and 102,638,388 shares outstanding, respectively Common Stock, Value, Issued Schedule of Information Related to Borrowings Schedule of Long-Term Debt Instruments [Table Text Block] Unsecured Management Company Revolver Unsecured Management Company Revolver [Member] Unsecured Management Company Revolver Line of Credit Line of Credit [Member] Restatement Determination Date Restatement Determination Date Borrowing capacity, fifth percentage Line Of Credit Facility, Borrowing Capacity, Fifth Percentage Line Of Credit Facility, Borrowing Capacity, Fifth Percentage Non-controlled/Non-affiliated Investment, Unaffiliated Issuer [Member] Investment, percentage of interest entitled to receive Investment, Percentage Of Interest Entitled To Receive Investment, Percentage Of Interest Entitled To Receive RailPros Parent, LLC RailPros Parent, LLC [Member] RailPros Parent, LLC SkyKnight Alpha SkyKnight Income Alpha, LLC [Member] SkyKnight Income Alpha, LLC Amendment Flag Foreside Financial Group, LLC Foreside Financial Group, LLC [Member] Foreside Financial Group, LLC ATM Offering At the Market Offering [Member] At the Market Offering Daxko Acquisition Corporation Daxko Acquisition Corporation [Member] Daxko Acquisition Corporation Net investment income before income taxes Net investment income Net Investment Income Investments [Domain] Investments [Domain] Unfunded Debt Securities, Delayed Draws Or Other Future Funding Commitments, Denominated Unfunded Debt Securities, Delayed Draws Or Other Future Funding Commitments, Denominated [Member] Unfunded Debt Securities, Delayed Draws Or Other Future Funding Commitments, Denominated Ciklum Inc. Ciklum Inc. [Member] Ciklum Inc. Management fee payable Increase (Decrease) in Management Fee Payable SBA Guaranteed Debentures, Issued March 23, 2016, Maturity Date March1, 2026 SBA Guaranteed Debentures, Issued March 23, 2016, Maturity Date March 1, 2026 [Member] SBA Guaranteed Debentures, Issued March 23, 2016, Maturity Date March 1, 2026 Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Transfers into Level III Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Number of incentive fee components Management And Service Fees, Incentive Rate, Number Of Components Management And Service Fees, Incentive Rate, Number Of Components Entity Address, State or Province PEO Total Compensation Amount PEO Total Compensation Amount Notional Amount Notional amount Derivative, Notional Amount Share Repurchase Program [Domain] Share Repurchase Program [Domain] RLG Holdings, LLC RLG Holdings, LLC [Member] RLG Holdings, LLC Weighted average shares of common stock outstanding - basic (in shares) Denominator for basic weighted average share (in shares) Weighted Average Number of Shares Outstanding, Basic Schedule of Management Fees and Incentive Fees Incurred by the Company Schedule of Related Party Transactions [Table Text Block] Schedule of Investments [Abstract] Investment Company, Nonconsolidated Subsidiary [Line Items] Investment Company, Nonconsolidated Subsidiary [Line Items] Quarterly Catch-Up Threshold Investment Management Agreement, Quarterly Catch-Up Threshold [Member] Investment Management Agreement, Quarterly Catch-Up Threshold Funded Debt Investments Funded Debt Securities [Member] Funded Debt Securities Adjustments to reconcile net (decrease) increase in net assets resulting from operations to net cash provided by operating activities: Adjustments To Reconcile Net Assets From Operations To Cash Provided by (Used in) Operating Activities [Abstract] Adjustments To Reconcile Net Assets From Operations To Cash Provided by (Used in) Operating Activities Average debt outstanding Long-Term Debt, Average Amount Outstanding Infogain Corporation Infogain Corporation [Member] Infogain Corporation CRCI Longhorn Holdings, Inc. CRCI Longhorn Holdings, Inc. [Member] CRCI Longhorn Holdings, Inc. Document Information [Line Items] Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Basis of accounting Basis of Accounting, Policy [Policy Text Block] Reorganized Careismatic Brands, LLC Reorganized Careismatic Brands, LLC [Member] Reorganized Careismatic Brands, LLC Class of Stock [Axis] SBA Guaranteed Debentures, Issued September 21, 2016, Maturity Date September 1, 2026 SBA Guaranteed Debentures, Issued September 21, 2016, Maturity Date September 1, 2026 [Member] SBA Guaranteed Debentures, Issued September 21, 2016, Maturity Date September 1, 2026 Gross redemptions Investments in and Advances to Affiliates, at Fair Value, Gross Reductions Maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Investments Schedule of Investments [Text Block] Payable to broker Due to Correspondent Brokers Symplr Software Intermediate Holdings, Inc. Symplr Software Intermediate Holdings, Inc. [Member] Symplr Software Intermediate Holdings, Inc. iCIMS, Inc. iCIMS, Inc. [Member] iCIMS, Inc. Total expenses Investment Income, Investment Expense Recent accounting standards updates New Accounting Pronouncements, Policy [Policy Text Block] Earnings (loss) per share Earnings Per Share, Policy [Policy Text Block] Litigation Case [Axis] Litigation Case [Axis] Non-Controlling Interest in NMNLC Noncontrolling Interest [Member] Amortization of deferred financing costs Amortization of financing costs Amortization of Debt Issuance Costs Unsecured Notes Unsecured Debt [Member] Receivable from affiliates Increase (Decrease) Due from Affiliates Net borrowings Borrowings balance Long-Term Debt Average net assets for the period Investment Company, Average Net Assets Equity and other Other Security Investments [Member] Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] Net Assets Equity [Text Block] Security Exchange Name January 2026 Notes January 2026 Notes [Member] January 2026 Notes Valuation Approach and Technique [Axis] Valuation Approach and Technique [Axis] Annualized Hurdle Rate Investment Management Agreement, Annualized Hurdle Rate [Member] Investment Management Agreement, Annualized Hurdle Rate Investment Type Investment Type Concentration Risk [Member] Investment Type Concentration Risk Net decrease in net assets Stockholders' Equity, Period Increase (Decrease) Schedule of Property Information for NMNLC Property, Plant, and Equipment [Table Text Block] Name Outstanding Recovery, Individual Name Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Consumer Services Consumer Services Consumer Services Sector [Member] Consumer Services Sector Geographical [Domain] Geographical [Domain] Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year [Member] Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Schedule of Investments [Table] Schedule of Investments [Table] Net change due to hedging activity Change in Unrealized Appreciation (Depreciation) Unrealized Gain (Loss) on Derivatives Ratio to average net assets: Investment Company, Financial Ratios [Abstract] Outstanding balance Long-Term Debt, Gross Asset Class [Domain] Asset Class [Domain] Related Party Transactions [Abstract] Deferred financing costs Debt, Policy [Policy Text Block] Compensation Amount Outstanding Recovery Compensation Amount Individual: Individual [Axis] Warrants Warrant [Member] Weighted average shares of common stock outstanding - diluted (in shares) Denominator for diluted weighted average share (in shares) Weighted Average Number of Shares Outstanding, Diluted Deca Dental Holdings LLC Deca Dental Holdings LLC [Member] Deca Dental Holdings LLC PetVet Care Centers, LLC PetVet Care Centers, LLC [Member] PetVet Care Centers, LLC Entity Emerging Growth Company Related Party Transaction [Line Items] Related Party Transaction [Line Items] Derivative instruments and hedging activities Derivatives, Policy [Policy Text Block] SBIC III New Mountain Finance SBIC III, L.P. [Member] New Mountain Finance SBIC III, L.P. Underlying Security Market Price Change Underlying Security Market Price Change, Percent Total expenses, before waivers/reimbursements Investment Company, Expense Ratio Before Fee Waiver And Reimbursements Investment Company, Expense Ratio Before Fee Waiver And Reimbursements Fullsteam Operations LLC Fullsteam Operations LLC [Member] Fullsteam Operations LLC October 2031 Tranche C Notes October 2031 Tranche C Notes [Member] October 2031 Tranche C Notes Estimated fair value, liability Derivative Liability, Subject to Master Netting Arrangement, before Offset Permian Trust Permian Trust [Member] Permian Trust Receivable from affiliates Other Receivables February 2029 Notes February 2029 Notes [Member] February 2029 Notes Treasury Stock at Cost Treasury Stock, Common [Member] KPSKY Acquisition Inc. KPSKY Acquisition Inc. [Member] KPSKY Acquisition Inc. Adelaide Borrower, LLC Adelaide Borrower, LLC [Member] Adelaide Borrower, LLC SLP IV NMFC Senior Loan Program IV LLC [Member] NMFC Senior Loan Program IV LLC Repayment of Unsecured Notes Repayments of Unsecured Debt Proceeds from Unsecured Notes Proceeds from Issuance of Unsecured Debt NM YI, LLC NM YI, LLC [Member] NM YI, LLC Investment Company, Net Assets [Roll Forward] Investment Company, Net Assets [Roll Forward] Organization, Consolidation and Presentation of Financial Statements [Abstract] Ambrosia Topco LLC Ambrosia Topco LLC [Member] Ambrosia Topco LLC AAH Topco, LLC AAH Topco, LLC [Member] AAH Topco, LLC Investment Company, Financial Highlights [Line Items] Investment Company, Financial Highlights [Line Items] Net investment income Investment Company, Investment Income (Loss) Ratio Incentive fee, excluding accrued capital gains incentive fees Incentive Fee Expense, Income-Based All Executive Categories All Executive Categories [Member] Valuation Approach and Technique [Domain] Valuation Approach and Technique [Domain] Pension Adjustments Service Cost Pension Adjustments Service Cost [Member] Repayments of credit facility Repayments of Long-Term Lines of Credit LogRhythm, Inc. LogRhythm, Inc. [Member] LogRhythm, Inc. Non-usage fee Line of Credit Facility, Commitment Fee Amount Asset coverage ratio Investment Company, Senior Security, Indebtedness, Asset Coverage Ratio Distributions Investment Company, Distribution [Policy Text Block] Accounting Changes and Error Corrections [Abstract] Schedule of Basic and Diluted Earnings (Loss) Per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Debt Disclosure [Abstract] Deferred financing costs, accumulated amortization Accumulated Amortization, Debt Issuance Costs Sale of stock, issuance amount Sale Of Stock, Maximum Issuance, Amount Sale Of Stock, Maximum Issuance, Amount Shares redeemed for a promissory note (in shares) Investment Owned, Shares Redeemed Investment Owned, Shares Redeemed Investment Company, Nonconsolidated Subsidiary [Domain] Investment Company, Nonconsolidated Subsidiary [Domain] Segment Reporting Segment Reporting [Text Block] Minimum Low Minimum [Member] Investment Type [Axis] Investment Type [Axis] Total investments Investments, Fair Value Disclosure Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Offering costs Adjustments to Additional Paid in Capital, Stock Offering Costs Adjustments to Additional Paid in Capital, Stock Offering Costs Liabilities Liabilities [Abstract] Convey Health Solutions, Inc. Convey Health Solutions, Inc. [Member] Convey Health Solutions, Inc. Income Statement Location [Axis] Location, Statement of Income, Balance [Axis] Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Quarterly Hurdle Rate Investment Management Agreement, Quarterly Hurdle Rate [Member] Investment Management Agreement, Quarterly Hurdle Rate Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table [Member] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Other liabilities Other liabilities Due to affiliates Other Liabilities SBA-guaranteed debentures Guaranteed Debt [Member] Guaranteed Debt Offering costs Offering costs Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs Investments at fair value that are non-qualifying assets as a percentage of total assets Investment Owned, At Fair Value, Non-Qualifying Assets, Percent Of Total Assets Investment Owned, At Fair Value, Non-Qualifying Assets, Percent Of Total Assets Common stock, authorized (in shares) Common Stock, Shares Authorized Sale of stock, number of shares issued (in shares) Sale of Stock, Number of Shares Issued in Transaction Dividend income Dividend Income, Operating, Paid in Cash Assets under management Assets under Management, Carrying Amount Exercise Price Award Exercise Price Stock repurchase program, authorized amount Share Repurchase Program, Authorized, Amount Document Quarterly Report Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested [Member] Unfunded Debt Investments Unfunded Debt Securities [Member] Unfunded Debt Securities Effect of foreign exchange rate changes on cash and cash equivalents Effect of Exchange Rate on Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Including Discontinued Operation Trinity Air Consultants Holdings Corporation Trinity Air Consultants Holdings Corporation [Member] Trinity Air Consultants Holdings Corporation Baker Tilly Advisory Group, LP Baker Tilly Advisory Group, LP [Member] Baker Tilly Advisory Group, LP Securities purchased under collateralized agreements to resell, cost Cost basis of securities purchased under collateralized agreements to resell Securities Purchased Under Agreements To Resell, Cost Securities Purchased Under Agreements To Resell, Cost Measurement input Investment Company, Investment Owned, Measurement Input Investments, cost Cost Investment Owned, Cost Gross additions Investments in and Advances to Affiliates, at Fair Value, Gross Additions Interest payable Interest Payable Repayment of long-term debt Repayments of Long-Term Debt Receivable from unsettled securities sold Receivable from unsettled securities sold Receivable, Investment, Sale Diligent Corporation Diligent Corporation [Member] Diligent Corporation Management fee Management fee Management Fee Expense Accrued capital gains incentive fees Incentive Fee Expense, Capital-Gain-Based Liability Derivative Liability [Abstract] Awards Close in Time to MNPI Disclosures, Table Awards Close in Time to MNPI Disclosures [Table Text Block] Financial Highlights Investment Company, Financial Highlights [Text Block] Equity Component [Domain] Equity Component [Domain] Administration Agreement Administration Agreement [Member] Administration Agreement Measurement Input Type [Domain] Measurement Input Type [Domain] Total gains or losses included in earnings: Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Cash paid for purchase of investments Payment for Purchase, Investment, Operating Activity Sale of stock, consideration received Sale of Stock, Consideration Received on Transaction Adoption Date Trading Arrangement Adoption Date Securities Financing Transaction [Line Items] Securities Financing Transaction [Line Items] Counterparty Name [Domain] Counterparty Name [Domain] Variable Rate Period [Domain] Variable Rate Period [Domain] Variable Rate Period [Domain] Title Trading Arrangement, Individual Title Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount Accounting Policies [Abstract] Restatement does not require Recovery Restatement Does Not Require Recovery [Text Block] Offering costs paid Payments of Stock Issuance Costs Reduced base management fee percentage Management And Service Fees, Base Rate, Target Under Waiver Agreement Management And Service Fees, Base Rate, Target Under Waiver Agreement Interest Income Interest income Interest Income, Operating Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Dividend reinvestment plan, share price percentage, threshold Dividend Reinvestment Plan, Share Price Percentage, Threshold Dividend Reinvestment Plan, Share Price Percentage, Threshold Payable to broker Increase (Decrease) Due To Correspondent Brokers Increase (Decrease) Due To Correspondent Brokers Net change in unrealized appreciation (depreciation): Investment Owned, Recognized Unrealized Appreciation (Depreciation), Net [Abstract] Investment Owned, Recognized Unrealized Appreciation (Depreciation), Net Derivative Instruments, Gain (Loss) [Line Items] Derivative Instruments, Gain (Loss) [Line Items] Less: management and incentive fees waived (See Note 5) Investment Company, Contractual Fee Waived Contributions related to non-controlling interest in NMNLC Noncontrolling Interest, Increase from Subsidiary Equity Issuance Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] Weighted Average Weighted Average Weighted Average [Member] All Adjustments to Compensation All Adjustments to Compensation [Member] Upfront Payments/Receipts Payments for (Proceeds from) Derivative Instrument, Financing Activities Indirect administrative expenses Investment Company, Fee Paid Indirectly Income taxes Income Tax, Policy [Policy Text Block] Net cash flows provided by operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation Energy Energy Energy Sector [Member] CG Group Holdings, LLC CG Group Holdings, LLC [Member] CG Group Holdings, LLC Income Statement [Abstract] Net assets Equity, Attributable to Parent [Abstract] Document Transition Report Variable Rate Period [Axis] Variable Rate Period [Axis] Variable Rate Period Non-cash dividend income Dividend Income, Operating, Paid in Kind Expenses Operating Expenses [Abstract] Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Fees waived by the administrator Investment Company, Voluntary Fee Waived Other Commitments [Domain] Other Commitments [Domain] Spread Investment, Basis Spread, Variable Rate City Area Code Document Information [Table] Subsequent Events Subsequent Events [Text Block] Name Forgone Recovery, Individual Name Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Entity [Domain] Non-cash activity on investments Noncash Activity On Investments Noncash Activity On Investments Per share market value (in dollars per share) Investment Company, Market Value, Per Share Preferred stock, issued (in shares) Preferred Stock, Shares Issued Ambrosia Holdco Corp. Ambrosia Holdco Corp. [Member] Ambrosia Holdco Corp. Investment Management Agreement Investment Management Agreement [Member] Investment Management Agreement Other assets Increase (Decrease) in Other Operating Assets Principal amount Principal Amount Principal amount Investment Owned, Balance, Principal Amount Derivative asset, subject to a master netting arrangement Derivative Asset, Subject to Master Netting Arrangement, Liability Offset Measurement Basis [Axis] Measurement Basis [Axis] Per share data: Investment Company, Per Share Data [Abstract] Investment Company, Per Share Data Cash and cash equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Mai Capital Management Intermediate LLC Mai Capital Management Intermediate LLC [Member] Mai Capital Management Intermediate LLC Fair Value Measurement Inputs and Valuation Techniques [Line Items] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Capital share activity Investment Company, Capital Share Transactions, Stock Issued [Abstract] Cash equivalents Cash Equivalents, at Carrying Value Subsequent Event Type [Domain] Subsequent Event Type [Domain] Weighted average interest rate Long-Term Debt, Weighted Average Interest Rate, over Time Stock repurchase program Stockholders' Equity, Policy [Policy Text Block] Investment Company [Abstract] Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Schedule of Unobservable Inputs Used in the Fair Value Measurement Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] Deferred financing costs paid Payments of Debt Issuance Costs Entity Current Reporting Status Net cash flows used in financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Controlled Investment, Affiliated Issuer, Controlled [Member] Total Shareholder Return Amount Total Shareholder Return Amount Total return based on net asset value Investment Company, Total Return Line of Credit Facility [Line Items] Line of Credit Facility [Line Items] Specialty Chemicals & Materials Specialty Chemicals & Materials Chemicals Sector [Member] IG Investments Holdings, LLC IG Investments Holdings, LLC [Member] IG Investments Holdings, LLC Fair value Property, Plant, and Equipment, Fair Value Disclosure Cash flows from financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Commitment fee percentage on unused capacity Line of Credit Facility, Unused Capacity, Commitment Fee Percentage Financial Instruments [Domain] Financial Instruments [Domain] Provision for taxes Provision for taxes Gain (Loss), Investment and Derivative, Operating, Tax Expense (Benefit) Deferred tax liability Increase (Decrease) In Deferred Income Tax Liability Increase (Decrease) In Deferred Income Tax Liability Other income Other Income Other income Other Income Derivative asset at fair value Derivative Asset Underlying Securities Award Underlying Securities Amount Fair Value Estimate of Fair Value Measurement [Member] Non-cash financing activities: Noncash Financing Items [Abstract] Noncash Financing Items Net Lease Net Lease Real Estate Sector [Member] Schedule of Financial Highlights Investment Company, Financial Highlights [Table Text Block] Healthcare Healthcare Healthcare Sector [Member] Schedule of Related Party Transactions, by Related Party [Table] Related Party Transaction [Table] American Achievement Corporation (aka AAC Holding Corp.) American Achievement Corporation (aka AAC Holding Corp.) [Member] American Achievement Corporation (aka AAC Holding Corp.) Commitments and Contingencies Disclosure [Abstract] All Trading Arrangements All Trading Arrangements [Member] Other Performance Measure, Amount Other Performance Measure, Amount Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Number of investments held Investment Company, Number Of Investments Held Investment Company, Number Of Investments Held Current Fiscal Year End Date Investment additional period Investment Company, Nonconsolidated Subsidiary, Investment Period, Extension Term Investment Company, Nonconsolidated Subsidiary, Investment Period, Extension Term Principal amount, shares (in shares) Investment Owned, Balance, Shares MRI Software LLC MRI Software LLC [Member] MRI Software LLC Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Financial Services & Technology Financial Services Sector [Member] Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year [Member] Distributions declared per share (in dollars per share) Common Stock, Dividends, Per Share, Declared Securities Act File Number DCA TopCo LP DCA TopCo LP [Member] DCA TopCo LP Agreement term Related Party Transaction, Term Related Party Transaction, Term Recovery of Erroneously Awarded Compensation Disclosure [Line Items] Measure: Measure [Axis] Extending Lenders Extending Lenders [Member] Extending Lenders Amortization of debt (premium) discount Amortization of premium Amortization of discount Amortization of Debt Discount (Premium) Treasury stock, common (in shares) Treasury Stock, Common, Shares TVG-Edmentum Holdings, LLC TVG-Edmentum Holdings, LLC, [Member] TVG-Edmentum Holdings, LLC, Increase (decrease) in net assets resulting from operations: Net Income (Loss), Including Portion Attributable to Noncontrolling Interest [Abstract] Foundational Education Group, Inc. Foundational Education Group, Inc. [Member] Foundational Education Group, Inc. Equity Components [Axis] Equity Components [Axis] Cash and other assets Cash And Other Assets Cash And Other Assets Income taxes received Income Tax Paid, after Refund Received Entity Interactive Data Current Less: Net increase in net assets resulting from operations related to non-controlling interest in New Mountain Net Lease Corporation Net Income (Loss) Attributable to Noncontrolling Interest Viper Bidco, Inc. Viper Bidco. Inc. [Member] Viper Bidco. Inc. Entity Small Business Property, Plant, and Equipment [Line Items] Property, Plant, and Equipment [Line Items] Fair Value Disclosure of Asset and Liability Not Measured at Fair Value [Table] Fair Value Disclosure of Asset and Liability Not Measured at Fair Value [Table] Distribution payable Distribution Payable MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] Proceeds from sale of investment Proceeds from Disposition of Investment, Operating Activity Document Type Purchase amount Debt Instrument, Repurchased Face Amount Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Net asset value per share of New Mountain Finance Corporation (in dollars per share) Net asset value, beginning balance (in dollars per share) Net asset value, ending balance (in dollars per share) Tennessee Bidco Limited Tennessee Bidco Limited [Member] Tennessee Bidco Limited First lien Debt Securities, First Lien [Member] Debt Securities, First Lien PIK Investment, Interest Rate, Paid in Kind Open Swap Contract, Identifier [Axis] Open Swap Contract, Identifier [Axis] Subsequent Event [Line Items] Subsequent Event [Line Items] Statement of Stockholders' Equity [Abstract] Local Phone Number (Increase) decrease in operating assets: Increase (Decrease) in Operating Assets [Abstract] Switzerland SWITZERLAND Commitments and contingencies (See Note 9) Commitments and Contingencies Packaging Packaging Container and Packaging Sector [Member] Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Line Of Credit, Class B Advance Line Of Credit, Class B Advance [Member] Line Of Credit, Class B Advance Distributions declared to stockholders from net investment income Distributions declared Investment Company, Dividend Distribution Executive Category: Executive Category [Axis] Floating rates Debt Securities, Floating Interest Rate [Member] Debt Securities, Floating Interest Rate Investment period Investment Company, Nonconsolidated Subsidiary, Investment Period Investment Company, Nonconsolidated Subsidiary, Investment Period Counterparty Name [Axis] Counterparty Name [Axis] Other Commitments [Table] Other Commitments [Table] Australia AUSTRALIA Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Schedule of Net Assets Investment Company, Change in Net Assets [Table Text Block] Supplemental disclosure of cash flow information Supplemental Cash Flow Information [Abstract] PEO Name PEO Name Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] SLP III NMFC Senior Loan Program III LLC [Member] NMFC Senior Loan Program III LLC Entity Tax Identification Number Other Commitments [Line Items] Other Commitments [Line Items] Kele Holdco, Inc. Kele Holdco, Inc. [Member] Kele Holdco, Inc. Revenue recognition Revenue [Policy Text Block] Management fee payable Management Fee Payable Entity Incorporation, State or Country Code Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Incentive fee percentage Management and Service Fees, Incentive Rate Derivative liability, fair value, offset against collateral, net Derivative Liability, Subject to Master Netting Arrangement, after Offset and Deduction Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year [Member] Formation and Business Purpose Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] SBA Guaranteed Debentures, Issued March 25, 2020, Maturity Date March, 1 2030 SBA Guaranteed Debentures, Issued March 25, 2020, Maturity Date March, 1 2030 One [Member] SBA Guaranteed Debentures, Issued March 25, 2020, Maturity Date March, 1 2030 One Debt Instrument [Axis] Subsequent Event [Table] Subsequent Event [Table] CFS Management, LLC CFS Management, LLC [Member] CFS Management, LLC Conversion of 2022 Convertible Notes to Shares (in shares) Conversion of 2022 Convertible Notes (in shares) Stock Issued During Period, Shares, Conversion of Convertible Securities Consumer Products Consumer Products Consumer Products Sector [Member] Consumer Products Sector Schedule of Carrying value of Hedged Assets and Liabilities Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Basic earnings (loss) per share (in dollars per share) Basic earnings (loss) per share (in dollars per share) Earnings Per Share, Basic Measurement Input Type [Axis] Measurement Input Type [Axis] Firebird Acquisition Corp, Inc. Firebird Acquisition Corp, Inc. [Member] Firebird Acquisition Corp, Inc. Insider Trading Arrangements [Line Items] Accumulated Net Realized (Losses) Gains Retained Earnings, Accumulated Net Realized Gain (Loss) [Member] Retained Earnings, Accumulated Net Realized Gain (Loss) Affiliate of Investment Adviser Affiliate Of Investment Adviser [Member] Affiliate Of Investment Adviser Coupa Holdings, LLC Coupa Holdings, LLC [Member] Coupa Holdings, LLC October 2028 Notes October 2028 Notes [Member] October 2028 Notes Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table [Member] Intercompany Note Intercompany Note [Member] Intercompany Note Unfunded commitments principal amount Investment Company, Financial Support to Investee Contractually Required, Not Provided, Amount Fair Value Measurement [Domain] Fair Value Measurement [Domain] Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] June 2027 Notes June 2027 Notes [Member] June 2027 Notes United Kingdom UNITED KINGDOM Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Measurement Inputs and Valuation Techniques [Table] Percent of Total Investments at Fair Value Concentration Risk, Percentage Equity Awards Adjustments Equity Awards Adjustments [Member] New Benevis Topco, LLC New Benevis Topco, LLC [Member] New Benevis Topco, LLC Annualized Catch-Up Threshold Investment Management Agreement, Annualized Catch-Up Threshold [Member] Investment Management Agreement, Annualized Catch-Up Threshold Hedging Relationship [Domain] Hedging Relationship [Domain] EBITDA as defined for middle market business Investment Company, Investment Owned, Investment Objective, Middle Market Business, EBITDA Investment Company, Investment Owned, Investment Objective, Middle Market Business, EBITDA Industry Sector [Domain] Industry Sector [Domain] Other liabilities Increase (Decrease) in Other Operating Liabilities Investment owned, net assets, percentage Investment Owned, Net Assets, Percentage Model N, Inc. Model N, Inc. [Member] Model N, Inc. Total expenses, net of waivers/reimbursements Investment Company, Expense Ratio Including Fee Waiver And Reimbursements Investment Company, Expense Ratio Including Fee Waiver And Reimbursements Borrowings Long-Term Debt [Text Block] Planview Parent, Inc. Planview Parent, Inc. [Member] Planview Parent, Inc. New Mountain Finance Corporation Total Net Assets of NMFC Parent [Member] Conversion of 2022 Convertible Notes to Shares Conversion of 2022 Convertible Notes to Shares Stock Issued During Period, Value, Conversion of Convertible Securities Cumulative Amount of Basis Adjustment Derivative, Cumulative Amount Of Basis Adjustment Derivative, Cumulative Amount Of Basis Adjustment Preferred stock, authorized (in shares) Preferred Stock, Shares Authorized Investments Excluding Drawn Revolvers Investments Excluding Drawn Revolvers [Member] Investments Excluding Drawn Revolvers Distributions declared to stockholders from net investment income (in dollars per share) Investment Company, Distribution to Shareholders, Per Share Hedging Designation [Domain] Hedging Designation [Domain] Share Repurchase Program [Axis] Share Repurchase Program [Axis] Financing Commitment Letter to Purchase Investments Financing Commitment Letter to Purchase Investments [Member] Financing Commitment Letter to Purchase Investments Class of Stock [Domain] Class of Stock [Domain] Statement of Cash Flows [Abstract] Other commitments Other Commitment Legends Hospitality Holding Company, LLC Legends Hospitality Holding Company, LLC [Member] Legends Hospitality Holding Company, LLC SBA Guaranteed Debentures, Issued March 25, 2015, Maturity Date March 1, 2025 SBA Guaranteed Debentures, Issued March 25, 2015, Maturity Date March 1, 2025 [Member] SBA Guaranteed Debentures, Issued March 25, 2015, Maturity Date March 1, 2025 Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Investments Investment, Policy [Policy Text Block] Non-cash investment income Other Noncash Income (Expense) Securities purchased under collateralized agreements to resell (cost of $30,000 and $30,000, respectively) Fair value of securities purchased under collateralized agreements to resell Securities purchased under agreements to resell Securities Purchased under Agreements to Resell Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Investment Company, Financial Highlights [Table] Investment Company, Financial Highlights [Table] Entity Filer Category Redemption of shares paid-in-kind Investment Company, Redemption of Shares, Paid-In-Kind Investment Company, Redemption of Shares, Paid-In-Kind Effective interest rate Debt Instrument, Interest Rate During Period Investments at Fair Value Investments, At Fair Value [Member] Investments, At Fair Value Flash Charm Inc. (fka Idera, Inc.) Flash Charm Inc. (fka Idera, Inc.) [Member] Flash Charm Inc. (fka Idera, Inc.) Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] HS Purchaser, LLC / Help/Systems Holdings, Inc. HS Purchaser, LLC / Help/Systems Holdings, Inc. [Member] HS Purchaser, LLC / Help/Systems Holdings, Inc. Not Designated as Hedging Instrument Not Designated as Hedging Instrument [Member] Net change in unrealized (depreciation) appreciation for the period relating to those Level III assets that were still held by the Company at the end of the period: Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) Holdings Credit Facility, Excluding Broadly Syndicated Loans Holdings Credit Facility, Excluding Broadly Syndicated Loans [Member] Holdings Credit Facility, Excluding Broadly Syndicated Loans Net increase (decrease) in net assets resulting from operations Net (decrease) increase in net assets resulting from operations Net increase (decrease) in net assets resulting from operations Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Total investment income Gross Investment Income, Operating Borrowing capacity, second percentage Line Of Credit Facility, Borrowing Capacity, Second Percentage Line Of Credit Facility, Borrowing Capacity, Second Percentage Derivative liability at fair value Fair Market Value Derivative Liability Foreign securities Foreign Currency Transactions and Translations Policy [Policy Text Block] Entity Address, Address Line One Total income tax provision Income Tax Expense (Benefit) Drawn Revolvers Drawn Revolvers [Member] Drawn Revolvers Fair Value Hedging Fair Value Hedging [Member] Derivative floating interest rate Derivative, Variable Interest Rate Investments in and Advances to Affiliates, at Fair Value [Roll Forward] Investments in and Advances to Affiliates, at Fair Value [Roll Forward] Net realized (losses) gains on investments and foreign currency Net Realized Gains (Losses) Realized Gain (Loss), Investment, Derivative, and Foreign Currency Transaction Price Change, Operating, before Tax Investments Including Delayed Draw Facilities Investments Including Delayed Draw Facilities [Member] Investments Including Delayed Draw Facilities New Permian Holdco, L.L.C. New Permian Holdco, L.L.C. [Member] New Permian Holdco, L.L.C. Portfolio turnover Investment Company, Portfolio Turnover Payment received for settlement Proceeds from Legal Settlements Recent Accounting Standards Updates Accounting Standards Update and Change in Accounting Principle [Text Block] Fixed rates Debt Securities, Fixed Interest Rate [Member] Debt Securities, Fixed Interest Rate Statement [Table] Statement [Table] Education Education Education Sector [Member] Education Sector Proceeds from sales and paydowns of investments Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales and Settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales and Settlements Conversion of 2022 Convertible Notes to Shares Debt Conversion, Converted Instrument, Amount Jersey JERSEY Estimated fair value, asset Derivative Asset, Subject to Master Netting Arrangement, before Offset Derivative variable interest rate Derivative, Basis Spread on Variable Rate Foreign currency Net change in unrealized depreciation (appreciation) on translation of assets and liabilities in foreign currencies Unrealized Gain (Loss), Foreign Currency Transaction, before Tax Damages sought Loss Contingency, Damages Sought, Value SBIC I New Mountain Finance SBIC, L.P. [Member] New Mountain Finance SBIC, L.P. Summary of Significant Accounting Policies Basis of Presentation and Significant Accounting Policies [Text Block]