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SMBC Capital Markets, Inc. [Member] SMBC Capital Markets, Inc. 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Total Compensation Amount Investment Advisory Agreement Investment Advisory Agreement [Member] Investment Advisory Agreement Assets, at fair value: Assets, Fair Value Disclosure [Abstract] Auto Components Auto Components Sector [Member] Auto Components Sector Senior secured Senior secured loans Debt Securities, Senior Secured Loan [Member] Debt Securities, Senior Secured Loan Share Repurchase Program [Domain] Share Repurchase Program [Domain] Notional amount Derivative, Notional Amount Basic weighted average common shares outstanding (Note 10) (in shares) Basic weighted average shares outstanding (in shares) Weighted Average Number of Shares Outstanding, Basic Proceeds from (fundings of) revolving loans, net Proceeds From (Purchases For) Undrawn Portion Of Revolving Loans, Operating Activities Proceeds From (Purchases For) Undrawn Portion Of Revolving Loans, Operating Activities Asset coverage ratio per unit (in dollars per share) Investment Company, Senior Security, Indebtedness, Asset Coverage Ratio Per Unit Investment Company, Senior Security, Indebtedness, Asset Coverage Ratio Per Unit Loss Contingencies [Line Items] Loss Contingencies [Line Items] Fair value non-accrual loan Fair Value, Option, Loans Held as Assets, Aggregate Amount in Nonaccrual Status Schedule of Investments [Abstract] Schedule of Investments [Abstract] SONIA Sterling Overnight Index Average (SONIA) [Member] Sterling Overnight Index Average (SONIA) Interest receivable Increase (Decrease) in Accrued Interest Receivable, Net Management and income incentive fees payable Management And Incentive Fee Payable Management And Incentive Fee Payable Food & Staples Retailing Food And Staples Retailing Sector [Member] Food And Staples Retailing Sector Canadian Overnight Repo Rate Average Canadian Overnight Repo Rate Average [Member] Canadian Overnight Repo Rate Average Debt instrument, received and retained, amount Debt Instrument, Received And Retained, Amount Debt Instrument, Received And Retained, Amount Average outstanding balance Average debt outstanding Long-Term Debt, Average Amount Outstanding Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Basis of presentation Basis of Accounting, Policy [Policy Text Block] PIK interest and non-cash dividends Fair Value, Measurement With Unobservable Inputs Reconciliation, Recurring Basis, Asset, Paid-In-Kind Interest And Non-Cash Dividends Fair Value, Measurement With Unobservable Inputs Reconciliation, Recurring Basis, Asset, Paid-In-Kind Interest And Non-Cash Dividends Capital Gain Incentive Fee Base Investment Advisory Agreement, Capital Gain Incentive Fee Base [Member] Investment Advisory Agreement, Capital Gain Incentive Fee Base Reconciliation of cash and cash equivalents, foreign currencies and restricted cash and cash equivalents and restricted foreign currencies Additional Cash Flow Elements and Supplemental Cash Flow Information [Abstract] Stock issued in connection with dividend reinvestment plan Stock Issued During Period, Value, Dividend Reinvestment Plan Gross reductions Investments in and Advances to Affiliates, at Fair Value, Gross Reductions O 2026 M6 Dividends O 2026 M6 Dividends [Member] O 2026 M6 Dividends Line of credit facility, maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Textiles, Apparel & Luxury Goods Textiles, Apparel & Luxury Goods Sector [Member] Textiles, Apparel & Luxury Goods Sector Class A-2RR 2024 Notes AAA Class A-2RR 2024 Notes [Member] AAA Class A-2RR 2024 Notes AAA Class A Senior Secured Floating Rate Notes Due 2034 AAA Class A Senior Secured Floating Rate Notes Due 2034 [Member] AAA Class A Senior Secured Floating Rate Notes Due 2034 Total return based on average net asset value - annualized Investment Company, Total Return, Annualized Percentage Investment Company, Total Return, Annualized Percentage Total expenses Investment Income, Investment Expense Recent Accounting Updates New Accounting Pronouncements, Policy [Policy Text Block] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Amortization of deferred debt issuance costs Amortization of debt issuance costs Amortization of Debt Issuance Costs Multiline Retail Multiline Retail Sector [Member] Multiline Retail Sector Interest rate swaps Interest Rate Derivative Assets, at Fair Value Investment Management Agreement - Incentive Rate, Quarterly Hurdle Rate Investment Management Agreement - Incentive Rate, Quarterly Hurdle Rate [Member] Investment Management Agreement - Incentive Rate, Quarterly Hurdle Rate Stock issued in connection with dividend reinvestment plan (in shares) DRIP Shares Issued (in shares) Stock Issued During Period, Shares, Dividend Reinvestment Plan Debt less unamortized debt issuance costs Long-term debt Amount Outstanding Long-Term Debt Less Than 1 Year Long-Term Debt, Gross, Maturity, Less Than 1 Year Long-Term Debt, Gross, Maturity, Less Than 1 Year Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] Common Stock Transactions Equity [Text Block] Number of investment portfolio company Number Of Investment Portfolio Companies Number Of Investment Portfolio Companies Security Exchange Name Subsidiary or Equity Method Investee [Line Items] Subsidiary or Equity Method Investee [Line Items] Valuation Approach and Technique [Axis] Valuation Approach and Technique [Axis] AAA Class A Senior Secured Floating Rate Loans Due 2034 AAA Class A Senior Secured Floating Rate Loans Due 2034 [Member] AAA Class A Senior Secured Floating Rate Loans Due 2034 Debt Instrument, Difference Rate [Axis] Debt Instrument, Tranche [Axis] Debt Instrument, Tranche Total liabilities, at fair value: Liabilities, Fair Value Disclosure Net realized gain (loss) on extinguishment of debt (in dollars per share) Investment Company, Extinguishment Of Debt, Gain (Loss), Per Share, Net Of Tax Investment Company, Extinguishment Of Debt, Gain (Loss), Per Share, Net Of Tax Total increase (decrease) common stock, outstanding Stockholders' Equity, Period Increase (Decrease) Segment reporting Segment Reporting, Policy [Policy Text Block] Name Outstanding Recovery, Individual Name Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Investment Advisory Agreement, Incentive Fee Cap Investment Advisory Agreement, Incentive Fee Cap [Member] Investment Advisory Agreement, Incentive Fee Cap Collateral (Received) / Pledged Derivative Liability, Subject to Master Netting Arrangement, Collateral, Right to Reclaim Security Not Offset Airlines Airlines Sector [Member] Airlines 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] Proceeds from non-cash dividends Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Proceeds From Non-Cash Dividends Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Proceeds From Non-Cash Dividends Line of credit facility, commitment fee percentage Line Of Credit Facility, Aggregate Commitments Each Day, Percentage Line Of Credit Facility, Aggregate Commitments Each Day, Percentage Accretion of discounts and amortization of premiums on issued debt securities Accretion (Amortization) of Discounts and Premiums, Debt Securities Accretion (Amortization) of Discounts and Premiums, Debt Securities Forward currency contracts Net change in unrealized (appreciation) depreciation on interest rate swap and forward currency contracts Foreign exchange Unrealized Gain (Loss) on Derivatives Non-qualifying Asset Non-qualifying Asset [Member] Non-qualifying Asset Debt Carrying Value Debt Long-Term Debt, Gross Debt Investments Debt investments Debt Securities [Member] Asset Class [Domain] Asset Class [Domain] Related Party Transactions [Abstract] Deferred debt issuance costs Debt, Policy [Policy Text Block] Oil, Gas & Consumable Fuels Oil, Gas & Consumable Fuels Sector [Member] Oil, Gas & Consumable Fuels Sector Compensation Amount Outstanding Recovery Compensation Amount Individual: Individual [Axis] Diluted weighted average common shares outstanding (Note 10) (in shares) Diluted weighted average shares outstanding (in shares) Weighted Average Number of Shares Outstanding, Diluted Entity Emerging Growth Company Related Party Transaction [Line Items] Related Party Transaction [Line Items] Net realized gain (loss) from: Realized Gain (Loss), Investment, Derivative, and Foreign Currency Transaction Price Change, Operating, before Tax [Abstract] Derivative instruments Derivatives, Policy [Policy Text Block] Debt Securitization Type [Domain] Debt Securitization Type [Domain] Debt Securitization Type [Domain] Expenses Paid Expenses Paid [Member] Expenses Paid Underlying Security Market Price Change Underlying Security Market Price Change, Percent Derivatives Derivative Instruments and Hedging Activities Disclosure [Text Block] Industrial Conglomerates Industrial Conglomerates Sector [Member] Industrial Conglomerates Sector Money market funds, at carrying value Cash Equivalent, Money Market Fund, Carrying Value 2024 Debt Securitization Debt Securitization, 2024 [Member] Debt Securitization, 2024 Gross Amount of Recognized (Liabilities) Derivative Liability, Subject to Master Netting Arrangement, before Offset A Class C-1 2018 Notes A Class C-1 2018 Notes [Member] A Class C-1 2018 Notes Investment Company, Net Assets [Roll Forward] Investment Company, Net Assets [Roll Forward] Organization, Consolidation and Presentation of Financial Statements [Abstract] Currency to be purchased Derivative Asset, Notional Amount Investment Company, Financial Highlights [Line Items] Investment Company, Financial Highlights [Line Items] Ratio of net investment income - after tax to average net assets Investment Company, Investment Income (Loss) Ratio All Executive Categories All Executive Categories [Member] Valuation Approach and Technique [Domain] Valuation Approach and Technique [Domain] Accretion of discounts (premium) on notes issued Accretion Of Debt Discount Accretion Of Debt Discount Pension Adjustments Service Cost Pension Adjustments Service Cost [Member] Secured 2021 Notes Secured 2021 Notes [Member] Secured 2021 Notes Asset coverage ratio Investment Company, Senior Security, Indebtedness, Asset Coverage Ratio Dividends and distributions Investment Company, Distribution [Policy Text Block] Schedule of Computation of Net Increase in Net Assets Per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Debt Disclosure [Abstract] Minimum Minimum [Member] Investment Type [Axis] Investment Type [Axis] O 2025 M9 Dividends O 2025 M9 Dividends [Member] O 2025 M9 Dividends 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] Effect of foreign currency exchange rates Effect of Exchange Rate on Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Location, Statement of Income, Balance [Axis] Location, Statement of Income, Balance [Axis] Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Macquarie Bank Limited Macquarie Bank Limited [Member] Macquarie Bank Limited 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] Non-cash dividends capitalized Non-cash Dividends Capitalized Non-cash Dividends Capitalized 2029 Notes Notes Due 2029 [Member] Notes Due 2029 Common stock, shares authorized (in shares) Common Stock, Shares Authorized The 2025 Program The 2025 Program [Member] The 2025 Program Number of shares issued under equity distribution agreements (in shares) Total Number of Shares Issued (in shares) Sale of Stock, Number of Shares Issued in Transaction Dividend income Dividend Income, Operating, Paid in Cash Schedule of Carrying Values and Fair Values of Debt Schedule of Carrying Values and Estimated Fair Values of Debt Instruments [Table Text Block] Exercise Price Award Exercise Price Stock repurchase program, authorized amount Share Repurchase Program, Authorized, Amount Document Quarterly Report GBDC 3 2022 Debt Securitization GBDC 3 2022 Debt Securitization [Member] GBDC 3 2022 Debt Securitization 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] Household Durables Household Durables Sector [Member] Household Durables Sector Range (Weighted Average) Investment Company, Investment Owned, Measurement Input Amortized cost Amortized Cost Amortized Cost: Investment Owned, Cost Non-cash dividend income Dividend Income, Operating, Non-Cash Dividend Income, Operating, Non-Cash Gross additions Investments in and Advances to Affiliates, at Fair Value, Gross Additions Interest payable Interest Payable Repayments of debt Repayments of Long-Term Debt Receivable for investments Receivable, Investment, Sale Base management fee Management Fee Expense Less unamortized debt issuance costs Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net Construction & Engineering Construction & Engineering Sector [Member] Construction & Engineering Sector 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] Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, gain (loss) included in earnings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Fundings of investments Payment for Purchase, Investment, Operating Activity Management and income incentive fees payable Increase (Decrease) in Management and Incentive Fees Payable Consideration received Sale of Stock, Consideration Received on Transaction Adoption Date Trading Arrangement Adoption Date Electrical Equipment Electrical Equipment Sector [Member] Electrical Equipment Sector Gross Proceeds Proceeds from Issuance or Sale of Equity Counterparty Name [Domain] Counterparty Name [Domain] Road & Rail Road And Rail Sector [Member] Road And Rail Sector 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] Fair Value Investments, At Fair Value [Member] Investments, At Fair Value Underwriting Fees/Offering Expenses Payments of Stock Issuance Costs Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Open Forward Foreign Currency Contract [Line Items] Open Forward Foreign Currency Contract [Line Items] Distributions declared: Distribution Declared [Abstract] Distribution Declared Market comparable companies Valuation, Market Comparable Company [Member] Valuation, Market Comparable Company Letter of Credit Letter of Credit [Member] 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 [Member] All Adjustments to Compensation All Adjustments to Compensation [Member] Transportation Infrastructure Transportation Infrastructure Sector [Member] Transportation Infrastructure Sector Income taxes Income Tax, Policy [Policy Text Block] Net cash provided by (used in) operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation Income Statement [Abstract] Net Assets Equity, Attributable to Parent [Abstract] Document Transition Report Equity Distribution Agreement Plan 2023 Equity Distribution Agreement Plan 2023 [Member] Equity Distribution Agreement Plan 2023 Dividend income, operating, paid in kind Dividend Income, Operating, Paid in Kind Expenses Operating Expenses [Abstract] Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Pharmaceuticals Pharmaceuticals Sector [Member] Pharmaceuticals Sector Investment, basis spread, variable rate Investment, Basis Spread, Variable Rate City Area Code Principal Investment Owned, Face Amount Geographic Concentration Risk Geographic Concentration Risk [Member] Subsequent Events Subsequent Events [Text Block] Name Forgone Recovery, Individual Name The 2024 Program The 2024 Program [Member] The 2024 Program Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] BBB Class D 2021 Notes BBB Class D 2021 Notes [Member] BBB Class D 2021 Notes Mid-Atlantic US Mid-Atlantic [Member] US Mid-Atlantic Per share market value at end of period (in dollars per share) Investment Company, Market Value, Per Share Preferred stock, shares issued (in shares) Preferred Stock, Shares Issued Debt Instrument, Tranche [Domain] Debt Instrument, Tranche [Domain] Debt Instrument, Tranche [Domain] Distributions to stockholders: Investment Company, Dividend Distribution [Abstract] Interest, dividend and fee income Interest and Dividend Income, Operating Accounts payable and other liabilities Accounts Payable and Other Accrued Liabilities Other assets Increase (Decrease) in Other Operating Assets Issuance of common stock (in shares) Stock Issued During Period, Shares, New Issues Tranche one Debt Instrument, Tranche One [Member] Debt Instrument, Tranche One Investment Management Agreement - Incentive Rate, Pre-Incentive Fee Net Investment Income Below Catch-Up Investment Management Agreement - Incentive Rate, Pre-Incentive Fee Net Investment Income Below Catch-Up [Member] Investment Management Agreement - Incentive Rate, Pre-Incentive Fee Net Investment Income Below Catch-Up Schedule of Common Stock Issued Under ATM Plan Schedule of Sale of Stock by Subsidiary or Equity Method Investee Disclosure [Table Text Block] Investment owned, balance, principal amount Investment Owned, Balance, Principal Amount AA Class B-1 GCIC 2018 Notes AA Class B-1 GCIC 2018 Notes [Member] AA Class B-1 GCIC 2018 Notes Unrealized depreciation Unrealized depreciation Derivative Asset, Subject to Master Netting Arrangement, Liability Offset Investment company, premium prepayment fees Investment Company, Premium Prepayment Fees Investment Company, Premium Prepayment Fees Containers & Packaging Containers & Packaging Sector [Member] Containers & Packaging Sector Fair Value Measurement Inputs and Valuation Techniques [Line Items] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Cash equivalents Cash Equivalents, at Carrying Value Subsequent Event Type [Domain] Subsequent Event Type [Domain] Share repurchase plan and Deferred offering costs Stockholders' Equity, Policy [Policy Text Block] Investment Company [Abstract] Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Schedule of Quantitative Information about Level 3 Fair Value Measurements Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] Capitalized debt issuance costs Payments of Debt Issuance Costs GBDC 3 2021 Debt Securitization GBDC 3 2021 Debt Securitization [Member] GBDC 3 2021 Debt Securitization Entity Current Reporting Status Net cash provided by (used in) financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Controlled affiliate company investments Investment, Affiliated Issuer, Controlled [Member] Total Shareholder Return Amount Total Shareholder Return Amount Liabilities, at fair value: Liabilities, Fair Value Disclosure [Abstract] GBDC 3 2022-2 Debt Securitization GBDC 3 2022-2 Debt Securitization [Member] GBDC 3 2022-2 Debt Securitization Total return based on average net asset value Investment Company, Total Return Chemicals Chemicals Sector [Member] Minimum asset coverage ratio required Investment Company, Senior Security, Indebtedness, Asset Coverage Ratio, Minimum Threshold Investment Company, Senior Security, Indebtedness, Asset Coverage Ratio, Minimum Threshold Qualifying Asset Type [Domain] Qualifying Asset Type [Domain] Qualifying Asset Type [Domain] Cash flows from financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Financial Instruments [Domain] Financial Instruments [Domain] Net unrealized appreciation on derivatives Net amounts presented in the Consolidated Statements of Financial Condition Derivative Asset Class C GCIC 2018 Notes Class C GCIC 2018 Notes [Member] Class C GCIC 2018 Notes Underlying Securities Award Underlying Securities Amount Commercial Services & Supplies Commercial Services & Supplies Sector [Member] Commercial Services & Supplies Sector Issuance of common stock Stock Issued During Period, Value, New Issues Forward currency contracts Foreign Currency Contracts, Liability, Fair Value Disclosure Real Estate Management & Development Real Estate Sector [Member] Road & Rail Road & Rail Sector [Member] Road & Rail Sector Schedule of Financial Highlights Investment Company, Financial Highlights [Table Text Block] 3 – 5 Years Long-Term Debt, Gross, Maturity, 3 Through 5 Years Long-Term Debt, Gross, Maturity, 3 Through 5 Years Aerospace & Defense Aerospace And Defense Sector [Member] Aerospace And Defense Sector Related Party Transaction [Table] Related Party Transaction [Table] Class A-1R 2024 Notes AAA/AAA Class A -1R 2024 Notes [Member] AAA/AAA Class A -1R 2024 Notes Adviser Revolver Adviser Revolver [Member] Adviser Revolver 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 Investment owned, balance, shares (in shares) Investment Owned, Balance, Shares Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Leisure Products Leisure Products Sector [Member] Leisure Products Sector Derivative [Line Items] Derivative [Line Items] Repurchases of common stock, net of commission costs (in shares) Total number of shares repurchased (in shares) Total Number of Shares Repurchased (in shares) Stock Repurchased During Period, Shares 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] Dividends and distributions declared per common share (in dollars per share) Common Stock, Dividends, Per Share, Declared Securities Act File Number Regions Bank Regions Bank [Member] Regions Bank Net gain (loss) on investment transactions Gain (Loss), Investment, Derivative, and Foreign Currency Transaction Price Change, Operating, before Tax Debt instrument, issued, principal Debt Instrument, Issued, Principal Recovery of Erroneously Awarded Compensation Disclosure [Line Items] Payment-in-kind interest capitalized Payment-in-kind interest capitalized Interest Income, Operating, Paid-In-Kind, Capitalized Interest Income, Operating, Paid-In-Kind, Capitalized Measure: Measure [Axis] Amortization of debt premium Amortization of Debt Discount (Premium) Maximum percentage of gross sales, commission received, percentage Maximum Percentage Of Gross Sales, Commission Received, Percentage Maximum Percentage Of Gross Sales, Commission Received, Percentage Spain SPAIN Equity Components [Axis] Equity Components [Axis] Related Party Related Party [Member] One stop One stop loans Debt Securities, One Stop Loan [Member] Debt Securities, One Stop Loan Entity Interactive Data Current Variable Rate [Axis] Variable Rate [Axis] Entity Small Business A Class C-R 2024 Notes A Class C-R 2024 Notes [Member] A Class C-R 2024 Notes Wells Fargo Bank, N.A. Wells Fargo Bank, N.A. [Member] Wells Fargo Bank, N.A. 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] Germany GERMANY MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] Proceeds from principal payments and sales of portfolio investments Proceeds from Disposition of Investment, Operating Activity Document Type Denmark DENMARK Net realized gain (loss) on extinguishment of debt Net realized (gain) loss on extinguishment of debt Realized loss on extinguishment of debt Gain (Loss) on Extinguishment of Debt Subordinated debt and second lien loans Debt Securities, Subordinated Debt and Second Lien Loan [Member] Debt Securities, Subordinated Debt and Second Lien Loan Net asset value per common share (in dollars per share) Net asset value at beginning of period (in dollars per share) Net asset value at end of period (in dollars per share) Total investment owned, at fair value and money market funds, at carrying value Investment Owned, At Fair Value And Money Market Funds, At Carrying Value Investment Owned, At Fair Value And Money Market Funds, At Carrying Value Investment, interest rate, paid in kind Investment, Interest Rate, Paid in Kind Subsequent Event [Line Items] Subsequent Event [Line Items] Statement of Stockholders' Equity [Abstract] Borrowings on debt Proceeds from Issuance of Long-Term Debt Local Phone Number Canada CANADA Commitments and Contingencies (Note 8) Commitments and Contingencies Short-term debt, average outstanding amount Short-Term Debt, Average Outstanding Amount Containers & Packaging Container and Packaging Sector [Member] Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Accounting Standards Update and Change in Accounting Principle [Table] Accounting Standards Update and Change in Accounting Principle [Table] Distributions from distributable earnings Cash Distribution Investment Company, Dividend Distribution Executive Category: Executive Category [Axis] Total investment owned and money market funds, percent of net assets Investment Owned And Money Market Funds, Percent of Net Assets Investment Owned And Money Market Funds, Percent of Net Assets Asset Acquisition Business Combination [Policy Text Block] Counterparty Name [Axis] Counterparty Name [Axis] Hotels, Restaurants & Leisure Hotels, Restaurants And Leisure Sector [Member] Hotels, Restaurants And Leisure Sector Australia AUSTRALIA Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Supplemental disclosure of cash flow information: Supplemental Cash Flow Information [Abstract] Annualized average stated interest rate Debt, Annualized Weighted Average Interest Rate Debt, Annualized Weighted Average Interest Rate PEO Name PEO Name Money market funds, percent of net assets Money Market Funds, Percent Of Net Assets Money Market Funds, Percent Of Net Assets Electronic Equipment, Instruments & Components Electronic Equipment, Instruments & Components Sector [Member] Electronic Equipment, Instruments & Components Sector Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Entity Tax Identification Number Revenue recognition Revenue [Policy Text Block] Entity Incorporation, State or Country Code Base Rate Base Rate [Member] Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] Management and service fees, incentive rate Management and Service Fees, Incentive Rate Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Preferred equity securities aggregate amount in nonaccrual status Fair Value, Option, Preferred Equity Securities, Aggregate Amount In Nonaccrual Status Fair Value, Option, Preferred Equity Securities, Aggregate Amount In Nonaccrual Status 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] Israel ISRAEL Organization Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Debt Instrument [Axis] Subsequent Event [Table] Subsequent Event [Table] Capital Markets Capital Markets Sector [Member] Capital Markets Sector Net contractual interest rate swap expense Interest Expense, Derivative, Excluding Amortization, Including Adjustments Interest Expense, Derivative, Excluding Amortization, Including Adjustments GBDC 3 DB Credit Facility GBDC 3 DB Credit Facility [Member] GBDC 3 DB Credit Facility Biotechnology Biotechnology Sector [Member] Biotechnology Sector Schedule of Effect of Offsetting Derivative Contracts Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Basic earnings (loss) per common share (Note 10) (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] Cumulative Hedging Adjustment Hedged Liability, Fair Value Hedge, Cumulative Increase (Decrease) 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] Investment company, financial commitment to investee, future amount Investment Company, Financial Support to Investee Contractually Required, Not Provided, Amount Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Cover [Abstract] Purchases of common stock for dividend reinvestment plan Payments For Repurchase Of Common Stock For Dividend Reinvestment Plan Payments For Repurchase Of Common Stock For Dividend Reinvestment Plan United Kingdom UNITED KINGDOM Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value: Concentration Risk, Percentage Average price paid per share (in dollars per share) Average Price Paid Per Share (in dollars per share) Stock Repurchased During Period, Average Price Paid Per Share Stock Repurchased During Period, Average Price Paid Per Share Equity Awards Adjustments Equity Awards Adjustments [Member] BBB- Class D 2018 Notes BBB- Class D 2018 Notes [Member] BBB- Class D 2018 Notes Industry Sector [Domain] Industry Sector [Domain] Average U.S. Dollar notional outstanding Derivative, Average Notional Amount Derivative, Average Notional Amount Percentage of Net Assets Investment Owned, Net Assets, Percentage Net change in unrealized appreciation (depreciation) on investment transactions (in dollars per share) Investment Company, Gain (Loss) On Unrealized Appreciation (Depreciation), Per Share Investment Company, Gain (Loss) On Unrealized Appreciation (Depreciation), Per Share Cumulative Pre-Incentive Fee Net Investment Income Investment Advisory Agreement, Cumulative Pre-Incentive Fee Net Investment Income [Member] Investment Advisory Agreement, Cumulative Pre-Incentive Fee Net Investment Income Finland FINLAND General and administrative expenses General and Administrative Expense Adjustments to reconcile net increase (decrease) in net assets resulting from operations to net cash provided by (used in) operating activities: Adjustments to reconcile net increase (decrease) in net assets resulting from operations to net cash provided by (used in) operating activities: [Abstract] Adjustments To Reconcile Net Increase (Decrease) In Net Assets Resulting From Operations To Net Cash Provided By (Used In) Operating Activities: Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Electronic Equipment, Instruments & Components Electronic Equipment, Instruments And Components Sector [Member] Electronic Equipment, Instruments And Components Sector France FRANCE Healthcare Equipment & Supplies Heathcare Equipment And Supplies [Member] Heathcare Equipment And Supplies From net investment income - after tax (in dollars per share) Amount Per Share (in dollars per share) Amount per share (in dollars per share) Investment Company, Distribution to Shareholders, Per Share Average debt outstanding per share (in dollars per share) Long-Term Debt, Average Amount Outstanding, Per Share Long-Term Debt, Average Amount Outstanding, Per Share Textiles, Apparel & Luxury Goods Textiles, Apparel And Luxury Goods Sector [Member] Textiles, Apparel And Luxury Goods Sector Share Repurchase Program [Axis] Share Repurchase Program [Axis] Hedging Designation [Domain] Hedging Designation [Domain] Statement of Cash Flows [Abstract] Foreign exchange Forward currency contracts Foreign Exchange Forward [Member] Midwest US Midwest Region [Member] Debt Securitization Type [Axis] Debt Securitization Type [Axis] Debt Securitization Type Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Investment transactions Investment, Policy [Policy Text Block] Software Software Sector [Member] Software Sector Media Media Sector [Member] Media Sector AAA/AAA Class A-1 GCIC 2018 Notes AAA/AAA Class A-1 GCIC 2018 Notes [Member] AAA/AAA Class A-1 GCIC 2018 Notes Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Investment Company, Financial Highlights [Table] Investment Company, Financial Highlights [Table] Life Sciences Tools & Services Life Sciences Tools And Services Sector [Member] Life Sciences Tools And Services Sector Forward currency contracts Net realized (gain) loss on forward currency contracts Foreign exchange Gain (Loss) on Sale of Derivatives Entity Filer Category Dividends [Axis] Dividends [Axis] Subsidiary or Equity Method Investee, Sale of Stock [Table] Subsidiary or Equity Method Investee, Sale of Stock [Table] Average stated interest rate Debt Instrument, Interest Rate During Period Hotels, Restaurants & Leisure Hotels, Restaurants & Leisure Sector [Member] Hotels, Restaurants & Leisure Sector Luxembourg LUXEMBOURG Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Asset Concentration Risk Asset Concentration Risk [Member] Asset Concentration Risk Investments, Debt and Equity Securities [Abstract] Entity Address, Address Line Two Total investment income Gross Investment Income, Operating Net unrealized depreciation on derivatives Net amounts presented in the Consolidated Statements of Financial Condition Derivative Liability Other Short-Term Borrowings Other Short-Term Borrowings [Member] Other Short-Term Borrowings Senior Notes Senior Notes [Member] Foreign currency translation Foreign Currency Transactions and Translations Policy [Policy Text Block] Entity Address, Address Line One Investments in and Advances to Affiliates, at Fair Value [Roll Forward] Investments in and Advances to Affiliates, at Fair Value [Roll Forward] Net realized gain (loss) on investment transactions Net realized gain (loss) on investment transactions Realized Gain (Loss), Investment, Derivative, and Foreign Currency Transaction Price Change, Operating, before Tax Consolidation Consolidation, Policy [Policy Text Block] Portfolio turnover Investment Company, Portfolio Turnover Household Products Household Products Sector [Member] Household Products Sector Net increase (decrease) in net assets resulting from operations: Net Income (Loss) Attributable to Parent [Abstract] Diversified Telecommunication Services Diversified Telecommunication Services Sector [Member] Diversified Telecommunication Services Sector Statement [Table] Statement [Table] Qualifying Asset Type [Axis] Qualifying Asset Type [Axis] Qualifying Asset Type Healthcare Providers & Services Healthcare Providers And Services Sector [Member] Healthcare Providers And Services Sector Interest rate swaps Interest Rate Derivative Liabilities, at Fair Value Jersey JERSEY Gross Amount of Recognized Assets Derivative Asset, Subject to Master Netting Arrangement, before Offset Company pays Basis spread on variable rate Derivative, Basis Spread on Variable Rate Foreign currencies, cost Cash and Cash Equivalent, Held in Foreign Currency, Acquisition Cost Translation of assets and liabilities in foreign currencies Net change in unrealized (appreciation) depreciation on translation of assets and liabilities in foreign currencies Unrealized Gain (Loss), Foreign Currency Transaction, before Tax Significant Accounting Policies and Recent Accounting Updates Basis of Presentation and Significant Accounting Policies [Text Block]