FAIR VALUE HIERARCHY FOR INVESTMENTS—PORTFOLIO COMPOSITION - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2022 USD ($) company investment		Dec. 31, 2021 USD ($) company investment		Dec. 31, 2020 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Expenses allocated	$ 12,965		$ 10,277		$ 7,429
Achieved total return on investments	0.111		0.166
Fair value	$ 4,102,177	[1],[2]	$ 3,561,831	[3],[4]
Cost	$ 3,773,752	[2],[5]	$ 3,259,246	[4],[6]
Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Concentration risk, percentage	100.00%		100.00%
Investment Owned, At Cost | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Concentration risk, percentage	100.00%		100.00%
External Investment Manager | Affiliated Entity
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Expenses allocated	$ 13,000		$ 10,300		$ 7,400
Short-term portfolio investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of investments | investment	1		1
Short-term portfolio investments | Maximum | Debt investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment term	1 year
LMM portfolio investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of portfolio companies | company	78		73
Fair value	$ 2,060,500		$ 1,716,400
Cost	$ 1,719,900		$ 1,455,700
LMM portfolio investments | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Concentration risk, percentage	94.00%		81.00%
LMM portfolio investments | Debt investments | Investment Owned, At Cost | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Concentration risk, percentage	73.70%		70.90%
LMM portfolio investments | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Annual revenues	$ 10,000
Investments	$ 5,000
LMM portfolio investments | Minimum | Debt investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment term	5 years
LMM portfolio investments | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Annual revenues	$ 150,000
Investments	$ 75,000
LMM portfolio investments | Maximum | Debt investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment term	7 years
Private Loan portfolio investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of portfolio companies | company	85		75
Fair value	$ 1,471,500		$ 1,141,800
Cost	$ 1,500,300		$ 1,157,500
Private Loan portfolio investments | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Concentration risk, percentage	76.00%		60.00%
Private Loan portfolio investments | Debt investments | Investment Owned, At Cost | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Concentration risk, percentage	97.10%		95.70%
Private Loan portfolio investments | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 10,000
Private Loan portfolio investments | Minimum | Debt investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment term	3 years
Private Loan portfolio investments | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 75,000
Private Loan portfolio investments | Maximum | Debt investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment term	7 years
Middle Market portfolio investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of portfolio companies | company	31		36
Fair value	$ 329,100		$ 395,200
Cost	$ 401,700		$ 440,900
Middle Market portfolio investments | Debt investments | Investment Owned, At Cost | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Concentration risk, percentage	93.80%		93.30%
Middle Market portfolio investments | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Annual revenues	$ 150,000
Investments	$ 3,000
Middle Market portfolio investments | Minimum | Debt investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment term	3 years
Middle Market portfolio investments | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Annual revenues	$ 1,500,000
Investments	$ 25,000
Middle Market portfolio investments | Maximum | Debt investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment term	7 years
Other portfolio investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of portfolio companies | company	14		13
Fair value	$ 116,300		$ 166,100
Cost	$ 120,400		$ 173,700
Other portfolio investments | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Concentration risk, percentage	2.80%		4.70%
Other portfolio investments | Investment Owned, At Cost | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Concentration risk, percentage	3.20%		5.30%
Other portfolio investments | Minimum | Debt investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment term	5 years
Other portfolio investments | Maximum | Debt investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment term	10 years
External investment manager
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 122,900		$ 140,400
Cost	$ 29,500		$ 29,500
External investment manager | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Concentration risk, percentage	3.00%		3.90%
External investment manager | Investment Owned, At Cost | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Concentration risk, percentage	0.80%		0.90%

[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. Negative fair value is the result of the capitalized discount on the loan or the unfunded commitment being valued below par.

[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 Corporate Facility or SPV Facility (each as defined in Note B.5. — Deferred Financing Costs, and together the “Credit Facilities”) or in support of the SBA-guaranteed debentures issued by the Funds.

[3] 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.

[4] 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 Corporate Facility or in support of the SBA-guaranteed debentures issued by the Funds.

[5] Principal is net of repayments. Cost is net of repayments and accumulated unearned income. Negative cost is the result of the capitalized discount being greater than the principal amount outstanding on the loan.

[6] Principal is net of repayments. Cost is net of repayments and accumulated unearned income.