DEBT - SPV Facility Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2022		Dec. 31, 2021		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
ASSETS
Investments at fair value:	$ 4,102,177	[1],[2]	$ 3,561,831	[3],[4]
Investment, cost	3,773,752	[2],[5]	3,259,246	[4],[6]
Cash and cash equivalents	49,121		32,629
Interest and dividend receivable and other assets	82,731		56,488
Receivable for securities sold	381		35,125
Deferred financing costs (net of accumulated amortization of $141 as of December 31, 2022)	7,475		4,217
Deferred financing costs, accumulated amortization	10,603		9,462
Total assets	4,241,885		3,690,290
LIABILITIES
Debt	1,999,102		1,796,056
Accounts payable and other liabilities	52,092		40,469
Interest payable	16,580		14,926
Total liabilities	2,133,299		1,901,444
Commitments and contingencies (Note K)
NET ASSETS
Contributed capital	2,030,531		1,736,346
Total undistributed earnings	77,271		51,793
Total net assets	2,108,586		1,788,846		$ 1,514,767	$ 1,536,390	$ 1,476,049	$ 1,380,368	$ 1,201,481	$ 1,070,894	$ 939,982	$ 792,533
Total liabilities and net assets	4,241,885		3,690,290
MSCC Funding I, LLC
ASSETS
Cash and cash equivalents	10,838
Interest and dividend receivable and other assets	2,828
Accounts receivable from MSCC and its subsidiaries	556
Receivable for securities sold	369
Deferred financing costs (net of accumulated amortization of $141 as of December 31, 2022)	2,630
Deferred financing costs, accumulated amortization	141
Total assets	333,728
LIABILITIES
Debt	200,000
Accounts payable and other liabilities	112
Interest payable	1,272
Total liabilities	201,384
Commitments and contingencies (Note K)
NET ASSETS
Contributed capital	126,010
Total undistributed earnings	6,334
Total net assets	132,344
Total liabilities and net assets	333,728
Non‑Control/Non‑Affiliate investments
ASSETS
Investments at fair value:	1,780,646	[1],[7]	1,523,360	[3],[8]
Investment, cost	1,867,414	[5],[7]	$ 1,573,110	[6],[8]
Non‑Control/Non‑Affiliate investments | MSCC Funding I, LLC
ASSETS
Investments at fair value:	316,507
Investment, cost	$ 314,752

[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.

[7] Non-Control/Non-Affiliate investments are defined by the 1940 Act as investments that are neither Control investments nor Affiliate investments.

[8] Non-Control/Non-Affiliate investments are defined by the 1940 Act as investments that are neither Control investments nor Affiliate investments.