Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Trading Arrangement: Trading Arrangement [Axis] Maximum High Maximum [Member] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Title of 12(b) Security Subsequent Event Subsequent Event [Member] Principal Amount Reported Value Measurement [Member] Fair Market Value Derivative Assets (Liabilities), at Fair Value, Net Derivative Instruments, Gain (Loss) [Table] Derivative Instruments, Gain (Loss) [Table] Commitment fee percentage Guaranteed Debt, Commitment Fee Guaranteed Debt, Commitment Fee 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] Borrowings allowable per dollar of equity Debt Outstanding Allowable Per Dollar Of Equity Debt Outstanding Allowable Per Dollar Of Equity Accumulated Net Investment Income Retained Earnings, Accumulated Net Investment Income (Loss) [Member] Retained Earnings, Accumulated Net Investment Income (Loss) Planview Parent, Inc. Planview Parent, Inc. [Member] Planview Parent, Inc. Minimum asset coverage ratio Asset Coverage, Required Ratio, Minimum Asset Coverage, Required Ratio, Minimum Kele Holdco, Inc. Kele Holdco, Inc. [Member] Kele Holdco, Inc. Derivative Instrument [Axis] Derivative Instrument [Axis] Non-controlled/Affiliated Investment, Affiliated Issuer, Noncontrolled [Member] Per share data: Investment Company, Per Share Data [Abstract] Investment Company, Per Share Data Current income tax expense (benefit) Current Income Tax Expense (Benefit) Deferred tax liability Increase (Decrease) In Deferred Income Tax Liability Increase (Decrease) In Deferred Income Tax Liability Conversion price (in dollars per share) Debt Instrument, Convertible, Conversion Price 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 ACI Group Holdings, Inc. ACI Group Holdings, Inc. [Member] ACI Group Holdings, Inc. Entity Common Stock, Shares Outstanding Atlas AU Bidco Pty Ltd Atlas AU Bidco Pty Ltd [Member] Atlas AU Bidco Pty Ltd Number of portfolio companies Investment Company, Number Of Portfolio Companies Investment Company, Number Of Portfolio Companies Non-Extending Lenders Non-Extending Lenders [Member] Non-Extending Lenders SBA Annual Charge Debt Instrument, Annual Charge, Percentage Debt Instrument, Annual Charge, Percentage 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] Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share NM YI, LLC NM YI, LLC [Member] NM YI, LLC Amortization of purchase discount Accretion (Amortization) of Discounts and Premiums, Investments Ciklum Inc. Ciklum Inc. [Member] Ciklum Inc. Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Number of reportable segments Number of Reportable Segments TVG-Edmentum Holdings, LLC, TVG-Edmentum Holdings, LLC, [Member] TVG-Edmentum Holdings, LLC, After Reinvestment Period After Reinvestment Period [Member] After Reinvestment Period Schedule of Notional Amounts and Fair Values of the Company's Derivative Instruments Schedule of Derivative Instruments [Table Text Block] New Mountain Capital New Mountain Capital [Member] New Mountain Capital Entity Address, City or Town Carrying Value Derivative, Fair Value, Net Statement of Financial Position [Abstract] Number of investments held Investment Company, Number Of Investments Held Investment Company, Number Of Investments Held Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount Floating rates Debt Securities, Floating Interest Rate [Member] Debt Securities, Floating Interest Rate Level II Fair Value, Inputs, Level 2 [Member] Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [Text Block] 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 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] Second lien Debt Securities, Second Lien [Member] Debt Securities, Second Lien Tranche C Notes Tranche C Notes [Member] Tranche C Notes Use of estimates Use of Estimates, Policy [Policy Text Block] DT1 Midco Corp. DT1 Midco Corp [Member] DT1 Midco Corp Food & Beverage Food & Beverage Food and Beverage Sector [Member] Related Party [Axis] Related and Nonrelated Parties [Axis] $19,000 SBA-guaranteed debentures, Issued September 2019 USD 19,000 SBA Guaranteed Debentures, Issued September 2019 [Member] USD 19,000 SBA Guaranteed Debentures, Issued September 2019 Document Period End Date PPV Intermediate Holdings, LLC PPV Intermediate Holdings, LLC [Member] PPV Intermediate Holdings, 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] 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 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 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 GC Waves Holdings, Inc. GC Waves Holdings, Inc. [Member] GC Waves Holdings, Inc. Unfunded Debt Securities, Revolving Credit Facilities Unfunded Debt Securities, Revolving Credit Facilities [Member] Unfunded Debt Securities, Revolving Credit Facilities Entity Shell Company 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] Purchase amount per $1,000 principal amount Debt Instrument, Repurchase Amount, Per USD1000 Of Principal Debt Instrument, Repurchase Amount, Per USD1000 Of Principal Company Selected Measure Name Company Selected Measure Name Sale of Stock [Domain] Sale of Stock [Domain] Administration Agreement Administration Agreement [Member] Administration Agreement Derivative fixed interest rate Derivative, Fixed Interest Rate Management fee waived Less: management fee waiver Investment Company, Management Fee, Contractual Fee Waived Firebird Acquisition Corp, Inc. Firebird Acquisition Corp, Inc. [Member] Firebird Acquisition Corp, Inc. Earnings per share—diluted Earnings Per Share, Diluted [Abstract] Eclipse Buyer Inc. Eclipse Buyer Inc. [Member] Eclipse Buyer Inc. Credit Facility [Domain] Credit Facility [Domain] Cash flows from operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] 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 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 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] $51,000 SBA-guaranteed debentures, Issued September 2020 USD 51,000 SBA Guaranteed Debentures, Issued September 2020 [Member] USD 51,000 SBA Guaranteed Debentures, Issued September 2020 Award Timing Predetermined Award Timing Predetermined [Flag] Issuance discount percentage Guaranteed Debt, Issuance Discount Guaranteed Debt, Issuance Discount Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Ambrosia Topco LLC Ambrosia Topco LLC [Member] Ambrosia Topco LLC 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] Coupa Holdings, LLC Coupa Holdings, LLC [Member] Coupa Holdings, LLC Conversion rate Debt Instrument, Convertible, Conversion Ratio Total square feet Area of Real Estate Property 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 Summary of Investment Holdings [Line Items] Summary of Investment Holdings [Line Items] 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 Income Approach Valuation, Income Approach [Member] Net expenses Investment Company, Expense, after Reduction of Fee Waiver and Reimbursement CRCI Longhorn Holdings, Inc. CRCI Longhorn Holdings, Inc. [Member] CRCI Longhorn Holdings, Inc. Preferred stock, par value (in dollars per share) Preferred Stock, Par or Stated Value Per Share Borrowing capacity, fifth percentage Line Of Credit Facility, Borrowing Capacity, Fifth Percentage Line Of Credit Facility, Borrowing Capacity, Fifth Percentage Borrowings Long-Term Debt, Fair Value Debt to equity ratio, maximum Debt Instrument, Covenant, Debt To Equity Ratio, Maximum Debt Instrument, Covenant, Debt To Equity Ratio, Maximum SLP III NMFC Senior Loan Program III LLC [Member] NMFC Senior Loan Program III LLC Concentration Risk Type [Domain] Concentration Risk Type [Domain] Fair Value Disclosures [Abstract] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Market & income approach Valuation, Market And Income Approach [Member] Valuation, Market And Income Approach Credit facility Credit facility outstanding balance Long-Term Line of Credit Meta Buyer LLC Meta Buyer LLC [Member] Meta Buyer LLC Distributions paid Payments of Ordinary Dividends, Common Stock 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 Hedging Relationship [Axis] Hedging Relationship [Axis] Regulation Regulatory Capital Requirements For Investment Companies [Text Block] Regulatory Capital Requirements For Investment Companies 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 Net assets at the end of the period Equity, Including Portion Attributable to Noncontrolling Interest Borrowing capacity, first percentage Line Of Credit Facility, Borrowing Capacity, First Percentage Line Of Credit Facility, Borrowing Capacity, First Percentage Subordinated Subordinated Debt [Member] Affiliates Affiliated Entity [Member] Healthspan Buyer, LLC Healthspan Buyer, LLC [Member] Healthspan Buyer, LLC Unsecured Notes Unsecured notes outstanding balance Unsecured Debt 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 Annualized Catch-Up Threshold Investment Management Agreement, Annualized Catch-Up Threshold [Member] Investment Management Agreement, Annualized Catch-Up Threshold Related Party Transaction [Axis] Related Party Transaction [Axis] Notorious Topco, LLC Notorious Topco, LLC [Member] Notorious Topco, LLC Asset Derivative Asset [Abstract] Borrowing capacity, fourth percentage Line Of Credit Facility, Borrowing Capacity, Fourth Percentage Line Of Credit Facility, Borrowing Capacity, Fourth Percentage Interest rate swaps Interest Rate Swap [Member] $4,000 SBA-guaranteed debentures, Issued September 2016 USD 4,000 SBA Guaranteed Debentures, Issued September 2016 [Member] USD 4,000 SBA Guaranteed Debentures, Issued September 2016 Deferred financing costs (net of accumulated amortization of $48,929 and $45,302, respectively) Deferred financing costs (net of accumulated amortization) Debt Issuance Costs, Net Consumer Services Consumer Services Consumer Services Sector [Member] Consumer Services Sector PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Community Management Holdings MidCo 2, LLC Community Management Holdings MidCo 2, LLC [Member] Community Management Holdings MidCo 2, LLC Unearned interest income Investment Owned, Unearned Interest Income Investment Owned, Unearned Interest Income 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 Business Products Business Products Business Products Sector [Member] Business Products Sector Stated interest rate Debt Instrument, Interest Rate, Stated Percentage Total net assets of New Mountain Finance Corporation Equity, Attributable to Parent Investment Company, Nonconsolidated Subsidiary [Table] Investment Company, Nonconsolidated Subsidiary [Table] Incentive fee payable Incentive Fee Payable Increases in dividend in excess (in dollars per share) Debt Instrument, Convertible, Dividends In Excess, Increase Debt Instrument, Convertible, Dividends In Excess, Increase Business Services Business Services Business Services Sector [Member] Business Services Sector Assets Assets [Abstract] EBITDA multiple Measurement Input, EBITDA Multiple [Member] Average weighted shares outstanding for the period (in shares) Weighted Average Number Of Shares Outstanding Weighted Average Number Of Shares Outstanding Mai Capital Management Intermediate LLC Mai Capital Management Intermediate LLC [Member] Mai Capital Management Intermediate LLC Stock repurchase program, additional authorized amount Share Repurchase Program, Authorized, Amount, Increase Share Repurchase Program, Authorized, Amount, Increase Professional fees Professional Fees Insider Trading Policies and Procedures [Line Items] Borrowing capacity, second percentage Line Of Credit Facility, Borrowing Capacity, Second Percentage Line Of Credit Facility, Borrowing Capacity, Second Percentage Investment, Issuer Affiliation [Domain] Investment, Issuer Affiliation [Domain] Financing Commitment Letter to Purchase Investments Financing Commitment Letter to Purchase Investments [Member] Financing Commitment Letter to Purchase Investments Deferred income tax (benefit) provision Deferred Income Tax Expense (Benefit) Net change in unrealized appreciation (depreciation): Investment Owned, Recognized Unrealized Appreciation (Depreciation), Net [Abstract] Investment Owned, Recognized Unrealized Appreciation (Depreciation), Net 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] Maximum purchase amount Debt Instrument, Repurchase Face Amount, Maximum Debt Instrument, Repurchase Face Amount, Maximum 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 (Benefit), Deferred Income Tax (Benefit) Provision and Total Income Tax (Benefit) 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 depreciation of investments Debt and Equity Securities, Unrealized Gain (Loss) Black Elk Trustee Black Elk Trustee [Member] Black Elk Trustee Basis spread on variable rate Debt Instrument, Basis Spread on Variable Rate SBA-guaranteed debentures SBA-guaranteed debentures outstanding Guaranteed Debt Guaranteed Debt All Individuals All Individuals [Member] 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) gains on investments Debt and Equity Securities, Realized Gain (Loss) Statement [Line Items] Statement [Line Items] Total assets Assets Interest and other financing expenses Interest and other financing expenses Interest Expense, Operating Consumer Products Consumer Products Consumer Products Sector [Member] Consumer Products Sector Industry Sector [Axis] Industry Sector [Axis] Convertible Notes Convertible Debt [Member] Extending Lenders Extending Lenders [Member] Extending Lenders Viper Bidco. Inc. Viper Bidco. Inc. [Member] Viper Bidco. Inc. PEO PEO [Member] Beginning of the period (in shares) End of the period (in shares) Shares, Outstanding Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Maximum secured debt ratio Debt Instrument, Covenant, Secured Debt Ratio, Maximum Debt Instrument, Covenant, Secured Debt Ratio, Maximum 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 Symplr Software Intermediate Holdings, Inc. Symplr Software Intermediate Holdings, Inc. [Member] Symplr Software Intermediate Holdings, Inc. 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 February 2029 Notes February 2029 Notes [Member] February 2029 Notes Line Of Credit, Class B Advance Line Of Credit, Class B Advance [Member] Line Of Credit, Class B Advance Proceeds from credit facility Proceeds from Long-Term Lines of Credit HS Purchaser, LLC / Help/Systems Holdings, Inc. HS Purchaser, LLC / Help/Systems Holdings, Inc. [Member] HS Purchaser, LLC / Help/Systems Holdings, Inc. Investment period Investment Company, Nonconsolidated Subsidiary, Investment Period Investment Company, Nonconsolidated Subsidiary, Investment Period $24,000 SBA-guaranteed debentures, Issued March 2020 USD 24,000 SBA Guaranteed Debentures, Issued March 2020 [Member] USD 24,000 SBA Guaranteed Debentures, Issued March 2020 Fixed rates Debt Securities, Fixed Interest Rate [Member] Debt Securities, Fixed Interest Rate Investment additional period Investment Company, Nonconsolidated Subsidiary, Investment Period, Extension Term Investment Company, Nonconsolidated Subsidiary, Investment Period, Extension Term Repayment of SBA-guaranteed debentures Repayment of SBA-guaranteed debentures Repayments of Other Long-Term Debt Total return based on market value Investment Company, Total Return, Market Value April 2024 Notes April 2024 Notes [Member] April 2024 Notes Variable Rate Period [Axis] Variable Rate Period [Axis] Variable Rate Period Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Interest payable Increase (Decrease) in Interest Payable, Net SBA-guaranteed debentures Guaranteed Debt [Member] Guaranteed Debt SBIC II New Mountain Finance SBIC II, L.P. [Member] New Mountain Finance SBIC II, L.P. 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] Shares issued in debt conversion (in shares) Debt Conversion, Converted Instrument, Shares Issued Cumulative Amount of Basis Adjustment Derivative, Cumulative Amount Of Basis Adjustment Derivative, Cumulative Amount Of Basis Adjustment New Permian Holdco, L.L.C. New Permian Holdco, L.L.C. [Member] New Permian Holdco, L.L.C. Funded Debt Investments Funded Debt Securities [Member] Funded Debt Securities Sale of Stock [Axis] Sale of Stock [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 (Losses) gains recognized Derivative, Gain (Loss) on Derivative, Net Members' capital Members' Equity Related Party Transaction [Domain] Related Party Transaction [Domain] 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 Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] Investments at fair value Fair value, beginning balance Fair value, ending balance Fair Value Fair value Investment Owned, Fair Value Equity Equity Securities [Member] Administrative expenses Administrative Fees Expense Common stock, issued (in shares) Common Stock, Shares, Issued Beginning balance Ending balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value eResearchTechnology, Inc. eResearchTechnology, Inc. [Member] eResearchTechnology, Inc. $41,000 SBA-guaranteed debentures, Issued March 2020 USD 41,000 SBA Guaranteed Debentures, Issued March 2020 [Member] USD 41,000 SBA Guaranteed Debentures, Issued March 2020 Daxko Acquisition Corporation Daxko Acquisition Corporation [Member] Daxko Acquisition Corporation Repurchase of common stock (in dollars per share) Investment Company, Share Repurchase, Per Share 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 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] Fullsteam Operations LLC Fullsteam Operations LLC [Member] Fullsteam Operations LLC Quarterly Hurdle Rate Investment Management Agreement, Quarterly Hurdle Rate [Member] Investment Management Agreement, Quarterly Hurdle Rate Subsequent Events [Abstract] Aggregate principal amount converted Debt Conversion, Original Debt, Amount Fair Value, Recurring and Nonrecurring [Table] Fair Value, Recurring and Nonrecurring [Table] Investments Including Delayed Draw Facilities Investments Including Delayed Draw Facilities [Member] Investments Including Delayed Draw Facilities Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] New Benevis Topco, LLC New Benevis Topco, LLC [Member] New Benevis Topco, LLC Accrued but unpaid deferred financing costs Debt Issuance Costs Incurred but Not Yet Paid Debt Issuance Costs Incurred but Not Yet Paid November 2028 Notes November 2028 Notes [Member] November 2028 Notes 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 PetVet Care Centers, LLC PetVet Care Centers, LLC [Member] PetVet Care Centers, 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] Nelipak Holding Company Nelipak Holding Company [Member] Nelipak Holding Company June 2027 Notes June 2027 Notes [Member] June 2027 Notes Investments threshold period past due for nonaccrual status Investment Owned, Nonaccrual, Threshold Period Past Due Investment Owned, Nonaccrual, Threshold Period Past Due 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 Borrowing capacity, third percentage Line Of Credit Facility, Borrowing Capacity, Third Percentage Line Of Credit Facility, Borrowing Capacity, Third Percentage (Decrease) increase in operating liabilities: Increase (Decrease) in Operating Liabilities [Abstract] All Award Types Award Type [Domain] Fair Value Fair Value Disclosures [Text Block] Revenue multiple Measurement Input, Revenue Multiple [Member] Other Other Valuation Technique [Member] Other Valuation Technique Level III Fair Value, Inputs, Level 3 [Member] 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 Redemption of shares paid-in-kind Investment Company, Redemption of Shares, Paid-In-Kind Investment Company, Redemption of Shares, Paid-In-Kind Geographical [Axis] Geographical [Axis] Foundational Education Group, Inc. Foundational Education Group, Inc. [Member] Foundational Education Group, Inc. Incentive fee waived Less: incentive fee waiver Investment Company, Incentive Fee, Contractual Fee Waived Investment, Issuer Affiliation [Axis] Investment, Issuer Affiliation [Axis] Low Voltage Holdings Inc. Low Voltage Holdings Inc. [Member] Low Voltage Holdings Inc. Sale of stock, issuance amount Sale Of Stock, Maximum Issuance, Amount Sale Of Stock, Maximum Issuance, Amount Other assets Other Assets Legal Entity [Axis] Interest expense Interest Expense, Debt Total net incentive fee Investment Company, Inventive Fee Expense, After Reduction Of Fee Waiver Investment Company, Inventive Fee Expense, After Reduction Of Fee Waiver 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 Net Unrealized Appreciation (Depreciation) Retained Earnings, Net Unrealized Appreciation (Depreciation) [Member] Retained Earnings, Net Unrealized Appreciation (Depreciation) AAH Topco, LLC AAH Topco, LLC [Member] AAH Topco, LLC Non-NEOs Non-NEOs [Member] GS Acquisitionco, Inc. GS Acquisitionco, Inc. [Member] GS Acquisitionco, Inc. Long-Term Debt, Type [Domain] Long-Term Debt, Type [Domain] Bonterra LLC Bonterra LLC [Member] Bonterra LLC Repurchase of shares under the repurchase programs Payments for Repurchase of Common Stock Statistical Measurement [Axis] Statistical Measurement [Axis] Base management fee percentage Management and Service Fees, Base Rate Schedule of Effect of Hedging Derivative Instruments Derivative Instruments, Gain (Loss) [Table Text Block] Award Type Award Type [Axis] 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 appreciation (depreciation) 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) Education Education Education Sector [Member] Education Sector 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 Common Stock [Member] RLG Holdings, LLC RLG Holdings, LLC [Member] RLG Holdings, LLC Net realized (losses) gains: Net Realized Gains (Losses) [Abstract] Net Realized Gains (Losses) AWP Group Holdings, Inc. AWP Group Holdings, Inc. [Member] AWP Group Holdings, Inc. Non-cash activity on investments Noncash Activity On Investments Noncash Activity On Investments Derivative Contract [Domain] Derivative Contract [Domain] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Incentive fee payable Increase (Decrease) in Incentive Fee Payable Schedule of Information Related to Convertible Notes Convertible Debt [Table Text Block] Total liabilities and net assets Liabilities and Equity Total expenses, before waivers/reimbursements Investment Company, Expense Ratio Before Fee Waiver And Reimbursements Investment Company, Expense Ratio Before Fee Waiver And Reimbursements 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 Securities Financing Transaction [Table] Security Financing Transaction [Table] Non-controlling interest in New Mountain Net Lease Corporation Equity, Attributable to Noncontrolling Interest 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 2022 Convertible Notes 2022 Convertible Notes [Member] 2022 Convertible Notes 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 Realized Capital Gains Investment Management Agreement , Realized Capital Gains [Member] Investment Management Agreement , Realized Capital Gains 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] Ultimus Group Midco, LLC Ultimus Group Midco, LLC [Member] Ultimus Group Midco, LLC Sale of stock, price per share (in dollars per share) Sale of Stock, Price Per Share Investments Excluding Drawn Revolvers Investments Excluding Drawn Revolvers [Member] Investments Excluding Drawn Revolvers MRI Software LLC MRI Software LLC [Member] MRI Software LLC SBIC III New Mountain Finance SBIC III, L.P. [Member] New Mountain Finance SBIC III, L.P. SLP IV NMFC Senior Loan Program IV LLC [Member] NMFC Senior Loan Program IV LLC FS WhiteWater Borrower, LLC FS WhiteWater Borrower, LLC [Member] FS WhiteWater Borrower, LLC Tranche A Notes Tranche A Notes [Member] Tranche A Notes 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] Upfront fees percentage Guaranteed Debt, Upfront Fees Guaranteed Debt, Upfront Fees Structured Finance Obligations Structured Finance Obligations [Member] Structured Finance Obligations $13,000 SBA-guaranteed debentures, Issued September 2017 USD 13,000 SBA Guaranteed Debentures, Issued September 2017 [Member] USD 13,000 SBA Guaranteed Debentures, Issued September 2017 Net (decrease) increase in members' capital Increase (Decrease) In Member's Capital Increase (Decrease) In Member's Capital 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] Credit Facility [Axis] Credit Facility [Axis] Name Measure Name Interest expense recognized on derivative Interest Expense Recognized on Derivative Interest Expense Recognized on Derivative Pay vs Performance Disclosure [Line Items] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] During Reinvestment Period During Reinvestment Period [Member] During Reinvestment Period Debt Instrument [Line Items] Debt Instrument [Line Items] Paid in Capital in Excess of Par Additional Paid-in Capital [Member] NMFC Credit Facility NMFC Credit Facility [Member] NMFC Credit Facility TigerConnect, Inc. TigerConnect, Inc. [Member] TigerConnect, Inc. Name Trading Arrangement, Individual Name Cash and cash equivalents Cash and Cash Equivalent 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 Company Selected Measure Amount Company Selected Measure Amount Reorganized Careismatic Brands, LLC Reorganized Careismatic Brands, LLC [Member] Reorganized Careismatic Brands, LLC SkyKnight Alpha SkyKnight Income Alpha, LLC [Member] SkyKnight Income Alpha, LLC Non-PEO NEO Non-PEO NEO [Member] Net expense recognized on fair value hedge Gain (Loss) on Fair Value Hedges Recognized in Earnings Number of incentive fee components Management And Service Fees, Incentive Rate, Number Of Components Management And Service Fees, Incentive Rate, Number Of Components Income tax expense (benefit) 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] Drawn Revolvers Drawn Revolvers [Member] Drawn Revolvers 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 Interest and other financing expenses Interest Expense, Policy [Policy Text Block] 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 Investment income Revenues [Abstract] 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] Document Fiscal Year Focus Number of collateralized agreements Number Of Collateralized Agreements Number Of Collateralized Agreements 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] UniTek Global Services, Inc. UniTek Global Services, Inc. [Member] UniTek Global Services, Inc. Line of Credit Line of Credit [Member] Restatement Determination Date Restatement Determination Date Foreside Financial Group, LLC Foreside Financial Group, LLC [Member] Foreside Financial Group, LLC CG Group Holdings, LLC CG Group Holdings, LLC [Member] CG Group Holdings, LLC Non-controlled/Non-affiliated Investment, Unaffiliated Issuer [Member] Amendment Flag Jawbreaker Parent, Inc. Jawbreaker Parent, Inc. [Member] Jawbreaker Parent, Inc. Accumulated Net Realized (Losses) Gains Retained Earnings, Accumulated Net Realized Gain (Loss) [Member] Retained Earnings, Accumulated Net Realized Gain (Loss) Net investment income before income taxes Net investment income Net Investment Income Investments [Domain] Investments [Domain] Management fee payable Increase (Decrease) in Management Fee Payable Variable Rate Period [Domain] Variable Rate Period [Domain] Variable Rate Period [Domain] 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 Entity Address, State or Province Number of fee components Management And Service Fees, Number Of Components Management And Service Fees, Number Of Components PEO Total Compensation Amount PEO Total Compensation Amount Notional Amount Notional amount Derivative, Notional Amount Share Repurchase Program [Domain] Share Repurchase Program [Domain] 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] PDI TA Holdings, Inc. PDI TA Holdings, Inc. [Member] PDI TA Holdings, Inc. Investments at Fair Value Investments, At Fair Value [Member] Investments, At Fair Value Total expenses, net of waivers/reimbursements Investment Company, Expense Ratio Including Fee Waiver And Reimbursements Investment Company, Expense Ratio Including Fee Waiver And Reimbursements Average debt outstanding Long-Term Debt, Average Amount Outstanding Document Information [Line Items] Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Basis of accounting Basis of Accounting, Policy [Policy Text Block] Deca Dental Holdings LLC Deca Dental Holdings LLC [Member] Deca Dental Holdings LLC Class of Stock [Axis] Interest Rate Type Interest Rate Type Risk [Member] Interest Rate Type Risk 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 SkyKnight II SkyKnight Income II, LLC [Member] SkyKnight Income II, LLC 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 Trinity Air Consultants Holdings Corporation Trinity Air Consultants Holdings Corporation [Member] Trinity Air Consultants Holdings Corporation Unsecured Notes Unsecured Debt [Member] Permian Trust Permian Trust [Member] Permian Trust Receivable from affiliates Increase (Decrease) Due from Affiliates Net borrowings Borrowings balance Long-Term Debt Financial Services & Technology Financial Services And Technology Sector [Member] Financial Services And Technology Sector Average net assets for the period Investment Company, Average Net Assets Equity and other Other Security Investments [Member] New ACI Buyer LLC New ACI Buyer LLC [Member] New ACI Buyer LLC Payable to broker Increase (Decrease) Due To Correspondent Brokers Increase (Decrease) Due To Correspondent Brokers 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 DOCS, MSO, LLC DOCS, MSO, LLC [Member] DOCS, MSO, LLC Valuation Approach and Technique [Axis] Valuation Approach and Technique [Axis] October 2027 Notes October 2027 Notes [Member] October 2027 Notes 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] RailPros Parent, LLC RailPros Parent, LLC [Member] RailPros Parent, LLC Accrued but unpaid offering costs Stock Issuance Costs Incurred but Not Yet Paid Stock Issuance Costs Incurred but Not Yet Paid Adjustments to reconcile net increase in net assets resulting from operations to net cash (used in) 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 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] American Achievement Corporation (aka AAC Holding Corp.) American Achievement Corporation (aka AAC Holding Corp.) [Member] American Achievement Corporation (aka AAC Holding Corp.) Schedule of Investments [Table] Schedule of Investments [Table] Legends Hospitality Holding Company, LLC Legends Hospitality Holding Company, LLC [Member] Legends Hospitality Holding Company, LLC 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] Intercompany Revolver Intercompany Revolver [Member] Intercompany Revolver 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 Entity Emerging Growth Company Related Party Transaction [Line Items] Related Party Transaction [Line Items] Derivative instruments and hedging activities Derivatives, Policy [Policy Text Block] Underlying Security Market Price Change Underlying Security Market Price Change, Percent 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 Estimated fair value, liability Derivative Liability, Subject to Master Netting Arrangement, before Offset Receivable from affiliates Other Receivables Treasury Stock at Cost Treasury Stock, Common [Member] Repayment of Unsecured Notes Repayments of Unsecured Debt Promissory Note Promissory Note [Member] Promissory Note Investment Company, Net Assets [Roll Forward] Investment Company, Net Assets [Roll Forward] Organization, Consolidation and Presentation of Financial Statements [Abstract] Regulatory capital Small Business Association Regulation, Regulatory Capital, Actual Small Business Association Regulation, Regulatory Capital, Actual 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 Non-usage fee Line of Credit Facility, Commitment Fee Amount CFS Management, LLC CFS Management, LLC [Member] CFS Management, LLC Associations, Inc. Associations, Inc. [Member] Associations, Inc. Asset coverage ratio Investment Company, Senior Security, Indebtedness, Asset Coverage Ratio Distributions Investment Company, Distribution [Policy Text Block] Accounting Changes and Error Corrections [Abstract] DCA Investment Holding, LLC DCA Investment Holding, LLC [Member] DCA Investment Holding, LLC 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 Investment Company, Nonconsolidated Subsidiary [Domain] Investment Company, Nonconsolidated Subsidiary [Domain] Intercompany Note Intercompany Note [Member] Intercompany Note 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] Liabilities Liabilities [Abstract] Income Statement Location [Axis] Location, Statement of Income, Balance [Axis] Software Software Software Sector [Member] Software Sector Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] January 2026 Notes January 2026 Notes [Member] January 2026 Notes 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 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 Investment Type Investment Type Concentration Risk [Member] Investment Type Concentration Risk Offering costs Offering costs Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs Holdings Credit Facility Holdings Credit Facility [Member] Holdings Credit Facility Unearned income Investment Owned, Unearned Income Investment Owned, Unearned Income Common stock, authorized (in shares) Common Stock, Shares Authorized Cash and other assets Cash And Other Assets Cash And Other Assets 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 Tranche B Notes Tranche B Notes [Member] Tranche B Notes Assets under management Assets under Management, Carrying Amount Exercise Price Award Exercise Price Stock repurchase program, authorized amount Share Repurchase Program, Authorized, Amount Old Repurchase Program Old Repurchase Program [Member] Old Repurchase Program 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] 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 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 NC Topco, LLC NC Topco, LLC [Member] NC Topco, LLC Management fee Management fee Management Fee Expense Accrued capital gains incentive fees Incentive Fee Expense, Capital-Gain-Based Liability Derivative Liability [Abstract] 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 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] 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 Unfunded Debt Securities, Revolving Credit Facilities, Denominated Unfunded Debt Securities, Revolving Credit Facilities, Denominated [Member] Unfunded Debt Securities, Revolving Credit Facilities, Denominated Securities Financing Transaction [Line Items] Securities Financing Transaction [Line Items] Counterparty Name [Domain] Counterparty Name [Domain] Baker Tilly Advisory Group, LP Baker Tilly Advisory Group, LP [Member] Baker Tilly Advisory Group, LP Holdings Credit Facility, Excluding Broadly Syndicated Loans Holdings Credit Facility, Excluding Broadly Syndicated Loans [Member] Holdings Credit Facility, Excluding Broadly Syndicated Loans 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] Convey Health Solutions, Inc. Convey Health Solutions, Inc. [Member] Convey Health Solutions, Inc. Offering costs paid Payments of Stock Issuance Costs Interest Income Interest income Interest Income, Operating AmeriVet Partners Management, Inc. AmeriVet Partners Management, Inc. [Member] AmeriVet Partners Management, Inc. Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] 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 Total net management fee Investment Company, Management Fee Expense, After Reduction Of Fee Waiver Investment Company, Management Fee Expense, After Reduction Of Fee Waiver 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] Power Grid Holdings, Inc. Power Grid Holdings, Inc. [Member] Power Grid Holdings, Inc. 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] Income Statement [Abstract] Net assets Equity, Attributable to Parent [Abstract] Document Transition Report 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 Quarterly Catch-Up Threshold Investment Management Agreement, Quarterly Catch-Up Threshold [Member] Investment Management Agreement, Quarterly Catch-Up Threshold 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] Per share market value (in dollars per share) Investment Company, Market Value, Per Share Preferred stock, issued (in shares) Preferred Stock, Shares Issued Investment Management Agreement Investment Management Agreement [Member] Investment Management Agreement YLG Holdings, Inc. YLG Holdings, Inc. [Member] YLG Holdings, Inc. $2,000 SBA-guaranteed debentures, Issued September 2018 USD 2,000 SBA Guaranteed Debentures, Issued September 2018 [Member] USD 2,000 SBA Guaranteed Debentures, Issued September 2018 Unsecured Management Company Revolver Unsecured Management Company Revolver [Member] Unsecured Management Company Revolver Other assets Increase (Decrease) in Other Operating Assets Principal amount, par value Principal Amount or Par Value Principal amount Investment Owned, Balance, Principal Amount Unfunded Debt Securities, Bridge Facilities Unfunded Debt Securities, Bridge Facilities [Member] Unfunded Debt Securities, Bridge Facilities Derivative asset, subject to a master netting arrangement Derivative Asset, Subject to Master Netting Arrangement, Liability Offset Measurement Basis [Axis] Measurement Basis [Axis] Investment, percentage of interest entitled to receive Investment, Percentage Of Interest Entitled To Receive Investment, Percentage Of Interest Entitled To Receive Cash and cash equivalents Cash and Cash Equivalents, Policy [Policy Text Block] 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 Non-cash operating activities: Non Cash Operating And Investing Activities [Abstract] Non Cash Operating And Investing Activities Subsequent Event Type [Domain] Subsequent Event Type [Domain] Adelaide Borrower, LLC Adelaide Borrower, LLC [Member] Adelaide Borrower, LLC Weighted average interest rate Long-Term Debt, Weighted Average Interest Rate, over Time KPSKY Acquisition Inc. KPSKY Acquisition Inc. [Member] KPSKY Acquisition Inc. 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] Repurchase amount Investment Owned, Balance, Repurchase Amount Investment Owned, Balance, Repurchase Amount Specialty Chemicals & Materials Specialty Chemicals & Materials Chemicals Sector [Member] Unfunded Debt Investments Unfunded Debt Securities [Member] Unfunded Debt Securities 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 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.) Financial Instruments [Domain] Financial Instruments [Domain] Benefit (provision) for taxes Benefit (provision) for taxes Gain (Loss), Investment and Derivative, Operating, Tax Expense (Benefit) Other income Other Income Other income Other Income Derivative asset at fair value Fair Market Value Derivative Asset Flash Charm Inc. (fka Idera, Inc.) Flash Charm Inc. (fka Idera, Inc.) [Member] Flash Charm Inc. (fka Idera, Inc.) Underlying Securities Award Underlying Securities Amount Fair Value Estimate of Fair Value Measurement [Member] Net Lease Net Lease Real Estate Sector [Member] Funded Investments Funded Securities [Member] Funded Securities Schedule of Financial Highlights Investment Company, Financial Highlights [Table Text Block] OEConnection LLC OEConnection LLC [Member] OEConnection LLC 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 Healthcare Healthcare Healthcare Sector [Member] Schedule of Related Party Transactions, by Related Party [Table] Related Party Transaction [Table] 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] Current Fiscal Year End Date Principal amount, shares (in shares) Investment Owned, Balance, Shares Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Financial Services & Technology Financial Services Sector [Member] First lien Debt Securities, First Lien [Member] Debt Securities, First Lien 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 Infogain Corporation Infogain Corporation [Member] Infogain Corporation Line Of Credit, Class A Advance Line Of Credit, Class A Advance [Member] Line Of Credit, Class A Advance DCA TopCo LP DCA TopCo LP [Member] DCA TopCo LP Recovery of Erroneously Awarded Compensation Disclosure [Line Items] Measure: Measure [Axis] Einstein Parent, Inc. Einstein Parent, Inc. [Member] Einstein Parent, Inc. Amortization of debt discount (premium) Amortization of premium Amortization of discount Amortization of Debt Discount (Premium) Treasury stock, common (in shares) Treasury Stock, Common, Shares Increase (decrease) in net assets resulting from operations: Net Income (Loss), Including Portion Attributable to Noncontrolling Interest [Abstract] SBIC I New Mountain Finance SBIC, L.P. [Member] New Mountain Finance SBIC, L.P. 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 Equity Components [Axis] Equity Components [Axis] Income taxes paid (received) Income Tax Paid, after Refund Received Offering costs Adjustments to Additional Paid in Capital, Stock Offering Costs Adjustments to Additional Paid in Capital, Stock Offering Costs Unfunded Investments Unfunded Securities [Member] Unfunded Securities Entity Interactive Data Current Shares redeemed for a promissory note (in shares) Investment Owned, Shares Redeemed Investment Owned, Shares Redeemed 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 Reimbursements to administrators as a percentage of gross assets Investment Company, Fee Reimbursement To Gross Assets Investment Company, Fee Reimbursement To Gross Assets 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] Agreement term Related Party Transaction, Term Related Party Transaction, Term 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) 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 Affiliate of Investment Adviser Affiliate Of Investment Adviser [Member] Affiliate Of Investment Adviser (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] Industry Type Industry Concentration Risk [Member] Industry Concentration Risk TMK Hawk Parent, Corp. TMK Hawk Parent, Corp. [Member] TMK Hawk Parent, Corp. Distributions declared to stockholders from net investment income Distributions declared Investment Company, Dividend Distribution 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 Bullhorn, Inc. Bullhorn, Inc. [Member] Bullhorn, Inc. Executive Category: Executive Category [Axis] 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.) 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] Entity Tax Identification Number Other Commitments [Line Items] Other Commitments [Line Items] 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 $15,255 SBA-guaranteed debentures, Issued March 2018 USD 15,255 SBA Guaranteed Debentures, Issued March 2018 [Member] USD 15,255 SBA Guaranteed Debentures, Issued March 2018 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] Debt Instrument [Axis] Subsequent Event [Table] Subsequent Event [Table] 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 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] Insider Trading Arrangements [Line Items] Repurchase Program Repurchase Program [Member] Repurchase Program 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] 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] United Kingdom UNITED KINGDOM 2026A Notes 2026A Notes [Member] 2026A Notes Annualized Hurdle Rate Investment Management Agreement, Annualized Hurdle Rate [Member] Investment Management Agreement, Annualized Hurdle Rate 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] Non-cash financing activities: Noncash Financing Items [Abstract] Noncash Financing Items Hedging Relationship [Domain] Hedging Relationship [Domain] Dividend reinvestment plan, share price percentage, threshold Dividend Reinvestment Plan, Share Price Percentage, Threshold Dividend Reinvestment Plan, Share Price Percentage, Threshold Industry Sector [Domain] Industry Sector [Domain] Other liabilities Increase (Decrease) in Other Operating Liabilities Accretive effect of common stock issuances per share (in dollars per share) Investment Company, Accretive Effect Of Common Stock Issuances Per Share Investment Company, Accretive Effect Of Common Stock Issuances Per Share Investment owned, net assets, percentage Investment Owned, Net Assets, Percentage Borrowings Long-Term Debt [Text Block] New Mountain Finance Corporation Total Net Assets of NMFC Parent [Member] ATM Offering At the Market Offering [Member] At the Market Offering 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.) Preferred stock, authorized (in shares) Preferred Stock, Shares Authorized Distributions declared to stockholders from net investment income (in dollars per share) Investment Company, Distribution to Shareholders, Per Share Cardinal Parent, Inc. Cardinal Parent, Inc. [Member] Cardinal Parent, Inc. Hedging Designation [Domain] Hedging Designation [Domain] Share Repurchase Program [Axis] Share Repurchase Program [Axis] Class of Stock [Domain] Class of Stock [Domain] Statement of Cash Flows [Abstract] Other commitments Other Commitment IG Investments Holdings, LLC IG Investments Holdings, LLC [Member] IG Investments Holdings, LLC Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Investments Investment, Policy [Policy Text Block] New Benevis Holdco, Inc. New Benevis Holdco, Inc. [Member] New Benevis Holdco, Inc. Non-cash investment income Other Noncash Income (Expense) Shares sold (in shares) Investment Owned, Shares Sold Investment Owned, Shares Sold 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 Tennessee Bidco Limited Tennessee Bidco Limited [Member] Tennessee Bidco Limited Fortis Solutions Group, LLC Fortis Solutions Group, LLC [Member] Fortis Solutions Group, LLC Effective interest rate Debt Instrument, Interest Rate During Period New ACI Holdings LLC New ACI Holdings LLC [Member] New ACI Holdings LLC Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Model N, Inc. Model N, Inc. [Member] Model N, 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) 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 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 (benefit) provision Income Tax Expense (Benefit) 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 iCIMS, Inc. iCIMS, Inc. [Member] iCIMS, Inc. Portfolio turnover Investment Company, Portfolio Turnover Payment received for settlement Proceeds from Legal Settlements New Mountain Net Lease Corporation New Mountain Net Lease Corporation [Member] New Mountain Net Lease Corporation Recent Accounting Standards Updates Accounting Standards Update and Change in Accounting Principle [Text Block] Statement [Table] Statement [Table] 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 Diligent Corporation Diligent Corporation [Member] Diligent Corporation Damages sought Loss Contingency, Damages Sought, Value Summary of Significant Accounting Policies Basis of Presentation and Significant Accounting Policies [Text Block]