FAIR VALUE HIERARCHY FOR INVESTMENTS—PORTFOLIO COMPOSITION - Summary of Investments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2022 USD ($) company		Dec. 31, 2021 USD ($) company
Summary of Investment Holdings [Line Items]
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 Cost | Investment Type Concentration Risk
Summary of Investment Holdings [Line Items]
Concentration risk, percentage	100.00%		100.00%
Non-accrual Status | Investment Owned, At Cost | Investment Type Concentration Risk
Summary of Investment Holdings [Line Items]
Concentration risk, percentage	3.70%		3.30%
Equity investments | Investment Owned, At Cost | Investment Type Concentration Risk
Summary of Investment Holdings [Line Items]
Concentration risk, percentage	14.20%		16.20%
First lien debt | Investment Owned, At Cost | Investment Type Concentration Risk
Summary of Investment Holdings [Line Items]
Concentration risk, percentage	85.00%		82.50%
LMM portfolio investments
Summary of Investment Holdings [Line Items]
Number of portfolio companies | company	78		73
Fair value	$ 2,060,500		$ 1,716,400
Cost	1,719,900		1,455,700
Average EBITDA	$ 8,000		$ 6,200
Diluted equity ownership percentage	41.00%		40.00%
Investment company, number of companies within portfolio excluded from EBITDA calculation | company	3		3
LMM portfolio investments | Weighted Average
Summary of Investment Holdings [Line Items]
Weighted-average annual effective yield	12.30%		11.20%
LMM portfolio investments | Non-accrual Status | Weighted Average
Summary of Investment Holdings [Line Items]
Weighted-average annual effective yield	11.60%		10.60%
LMM portfolio investments | Debt investments | Investment Owned, At Cost | Investment Type Concentration Risk
Summary of Investment Holdings [Line Items]
Concentration risk, percentage	73.70%		70.90%
LMM portfolio investments | Equity investments | Investment Owned, At Cost | Investment Type Concentration Risk
Summary of Investment Holdings [Line Items]
Concentration risk, percentage	26.30%		29.10%
LMM portfolio investments | First lien debt | Investment Owned, At Cost | Investment Type Concentration Risk
Summary of Investment Holdings [Line Items]
Concentration risk, percentage	99.10%		99.00%
Private Loan portfolio investments
Summary of Investment Holdings [Line Items]
Number of portfolio companies | company	85		75
Fair value	$ 1,471,500		$ 1,141,800
Cost	1,500,300		1,157,500
Average EBITDA	$ 38,100		$ 41,300
Investment company, number of companies within portfolio excluded from EBITDA calculation | company	2		3
Private Loan portfolio investments | Weighted Average
Summary of Investment Holdings [Line Items]
Weighted-average annual effective yield	11.60%		8.20%
Private Loan portfolio investments | Non-accrual Status | Weighted Average
Summary of Investment Holdings [Line Items]
Weighted-average annual effective yield	11.20%		8.00%
Private Loan portfolio investments | Debt investments | Investment Owned, At Cost | Investment Type Concentration Risk
Summary of Investment Holdings [Line Items]
Concentration risk, percentage	97.10%		95.70%
Private Loan portfolio investments | Equity investments | Investment Owned, At Cost | Investment Type Concentration Risk
Summary of Investment Holdings [Line Items]
Concentration risk, percentage	2.90%		4.30%
Private Loan portfolio investments | First lien debt | Investment Owned, At Cost | Investment Type Concentration Risk
Summary of Investment Holdings [Line Items]
Concentration risk, percentage	99.60%		98.70%
Middle Market portfolio investments
Summary of Investment Holdings [Line Items]
Number of portfolio companies | company	31		36
Fair value	$ 329,100		$ 395,200
Cost	401,700		440,900
Average EBITDA	$ 68,700		$ 76,000
Investment company, number of companies within portfolio excluded from EBITDA calculation | company			1
Middle Market portfolio investments | Weighted Average
Summary of Investment Holdings [Line Items]
Weighted-average annual effective yield	11.00%		7.50%
Middle Market portfolio investments | Non-accrual Status | Weighted Average
Summary of Investment Holdings [Line Items]
Weighted-average annual effective yield	10.30%		6.90%
Middle Market portfolio investments | Debt investments | Investment Owned, At Cost | Investment Type Concentration Risk
Summary of Investment Holdings [Line Items]
Concentration risk, percentage	93.80%		93.30%
Middle Market portfolio investments | Equity investments | Investment Owned, At Cost | Investment Type Concentration Risk
Summary of Investment Holdings [Line Items]
Concentration risk, percentage	6.20%		6.70%
Middle Market portfolio investments | First lien debt | Investment Owned, At Cost | Investment Type Concentration Risk
Summary of Investment Holdings [Line Items]
Concentration risk, percentage	98.80%		98.70%

[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.