FAIR VALUE MEASUREMENTS - Schedule of Level 3 Input Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Total
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	$ 1,902,380	$ 1,796,303	$ 1,906,920	$ 1,835,094
Purchases	47,963	196,031	105,473	309,997
Sales	(34,150)	(91,355)	(72,745)	(107,417)
Paydowns	(72,255)	(69,443)	(102,037)	(212,824)
Accretion of discount	1,929	2,761	3,784	5,099
Balance, end of period	1,829,501	1,813,365	1,829,501	1,813,365
Net change in unrealized appreciation (depreciation) included in earnings related to investments still held at the reporting date included in net change in unrealized appreciation (depreciation) on investments on the Consolidated Statements of Operations	(7,148)	(22,181)	(4,099)	(22,329)
Total | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net gain (loss) included in earnings	(7,991)	54	(21,306)	5,893
Total | Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net gain (loss) included in earnings	(8,375)	(20,986)	9,412	(22,477)
First Lien Debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	1,350,740	1,224,117	1,359,962	1,232,084
Purchases	42,282	194,313	97,107	305,642
Sales	(32,655)	(86,311)	(64,656)	(98,370)
Paydowns	(72,255)	(69,443)	(100,976)	(175,416)
Accretion of discount	1,630	2,446	3,251	4,218
Balance, end of period	1,272,650	1,257,282	1,272,650	1,257,282
Net change in unrealized appreciation (depreciation) included in earnings related to investments still held at the reporting date included in net change in unrealized appreciation (depreciation) on investments on the Consolidated Statements of Operations	(7,830)	(8,529)	(10,746)	(11,587)
First Lien Debt | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net gain (loss) included in earnings	(9,180)	(504)	(22,609)	2,951
First Lien Debt | Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net gain (loss) included in earnings	(7,912)	(7,336)	571	(13,827)
Second Lien Debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	259,429	304,202	262,703	341,776
Purchases	4,051	431	4,374	681
Sales	0	(4,013)	(6,372)	(4,013)
Paydowns	0	0	(454)	(36,325)
Accretion of discount	241	182	419	741
Balance, end of period	265,536	290,683	265,536	290,683
Net change in unrealized appreciation (depreciation) included in earnings related to investments still held at the reporting date included in net change in unrealized appreciation (depreciation) on investments on the Consolidated Statements of Operations	1,814	(9,163)	1,814	(10,496)
Second Lien Debt | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net gain (loss) included in earnings	1	(956)	(48)	(956)
Second Lien Debt | Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net gain (loss) included in earnings	1,814	(9,163)	4,914	(11,221)
Equity Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	102,241	78,699	94,190	77,093
Purchases	1,630	1,287	3,992	3,674
Sales	(1,495)	(1,031)	(1,717)	(5,034)
Paydowns	0	0	(607)	(1,083)
Accretion of discount	58	133	114	140
Balance, end of period	102,214	78,633	102,214	78,633
Net change in unrealized appreciation (depreciation) included in earnings related to investments still held at the reporting date included in net change in unrealized appreciation (depreciation) on investments on the Consolidated Statements of Operations	(263)	(1,970)	5,797	(2,871)
Equity Investments | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net gain (loss) included in earnings	1,188	1,514	1,351	3,898
Equity Investments | Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net gain (loss) included in earnings	(1,408)	(1,969)	4,891	(55)
Subordinated Loan and Member's Interest
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	189,970	189,285	190,065	184,141
Purchases	0	0	0	0
Sales	0	0	0	0
Paydowns	0	0	0	0
Accretion of discount	0	0	0	0
Balance, end of period	189,101	186,767	189,101	186,767
Net change in unrealized appreciation (depreciation) included in earnings related to investments still held at the reporting date included in net change in unrealized appreciation (depreciation) on investments on the Consolidated Statements of Operations	(869)	(2,519)	(964)	2,625
Subordinated Loan and Member's Interest | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net gain (loss) included in earnings	0	0	0	0
Subordinated Loan and Member's Interest | Net change in unrealized appreciation (depreciation)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net gain (loss) included in earnings	$ (869)	$ (2,518)	$ (964)	$ 2,626