FAIR VALUE HIERARCHY FOR INVESTMENTS—PORTFOLIO COMPOSITION - Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021		Sep. 30, 2021	[2],[4],[5]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	$ 3,973,490	$ 3,561,831	[1],[2],[3]	$ 3,973,490
Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	3,973,490	3,561,831
Fair Value, Recurring | LMM portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	1,910,915	1,716,415
Fair Value, Recurring | Private Loan portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	1,476,934	1,141,772
Fair Value, Recurring | Middle Market portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	354,286	395,167
Fair Value, Recurring | Other Portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	117,010	166,083
Fair Value, Recurring | External Investment Manager investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	112,490	140,400
Fair Value, Recurring | Short-term portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	1,855	1,994
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring | LMM portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring | Private Loan portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring | Middle Market portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring | Other Portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring | External Investment Manager investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring | Short-term portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	0	0
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	1,855	1,994
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring | LMM portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	0	0
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring | Private Loan portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	0	0
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring | Middle Market portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	0	0
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring | Other Portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	0	0
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring | External Investment Manager investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	0	0
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring | Short-term portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	1,855	1,994
Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	3,971,635	3,559,837
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	3,971,635	3,559,837
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring | LMM portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	1,910,915	1,716,415
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring | Private Loan portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	1,476,934	1,141,772
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring | Middle Market portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	354,286	395,167
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring | Other Portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	117,010	166,083
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring | External Investment Manager investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	112,490	140,400
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring | Short-term portfolio investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value:	$ 0	$ 0

[1] Investment fair value was determined using significant unobservable inputs, unless otherwise noted. See Note C—Fair Value Hierarchy for Investments—Portfolio Composition for further discussion.

[2] All investments are Lower Middle Market portfolio investments, unless otherwise noted. See Note C—Fair Value Hierarchy for Investments—Portfolio Composition for a description of Lower Middle Market portfolio investments. All of the Company’s investments, unless otherwise noted, are encumbered either as security for the Company’s Credit Facility or in support of the SBA-guaranteed debentures issued by the Funds.

[3] All portfolio company headquarters are based in the United States, unless otherwise noted.

[4] Investment fair value was determined using significant unobservable inputs, unless otherwise noted. See Note C—Fair Value Hierarchy for Investments—Portfolio Composition for further discussion. Negative fair value is the result of the capitalized discount on the loan or the unfunded commitment being valued below par.

[5] All portfolio company headquarters are based in the United States, unless otherwise noted.