Fair Value of Investments - Schedule of Changes in the Fair Value of Investments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
First-lien senior secured debt investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	$ 9,276,243	$ 9,435,175	$ 9,279,179	$ 9,539,774
Purchases of investments, net	219,212	1,625,590	365,149	2,924,093
Payment-in-kind	42,671	27,417	70,253	44,953
Proceeds from investments, net	(643,017)	(1,773,839)	(784,653)	(3,152,864)
Net amortization/accretion of discount/premium on investments	8,168	10,094	15,625	20,281
Transfers between investment types	0		(47,819)
Transfers into (out of) Level 3	(13,005)	0	(13,371)	(514)
Fair value, end of period	8,885,986	9,242,688	8,885,986	9,242,688
First-lien senior secured debt investments | Net change in unrealized gain (loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in unrealized gain (loss) / realized gains (losses)	(4,404)	(81,700)	53,988	(133,211)
First-lien senior secured debt investments | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in unrealized gain (loss) / realized gains (losses)	118	(49)	(52,365)	176
Second-lien senior secured debt investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	1,761,897	1,780,538	1,817,286	1,921,447
Purchases of investments, net	0	193,947	(10)	300,964
Payment-in-kind	3,924	3,307	7,715	5,741
Proceeds from investments, net	(35,851)	(193,072)	(55,050)	(324,109)
Net amortization/accretion of discount/premium on investments	1,126	844	2,051	1,966
Transfers between investment types	0		0
Transfers into (out of) Level 3	(26,562)	34,545	(73,485)	(66,765)
Fair value, end of period	1,703,509	1,763,979	1,703,509	1,763,979
Second-lien senior secured debt investments | Net change in unrealized gain (loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in unrealized gain (loss) / realized gains (losses)	(1,025)	(56,130)	5,002	(75,265)
Second-lien senior secured debt investments | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in unrealized gain (loss) / realized gains (losses)	0	0	0	0
Unsecured debt investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	262,191	266,195	237,440	196,485
Purchases of investments, net	0	19,849	0	89,239
Payment-in-kind	5,259	2,916	13,458	8,778
Proceeds from investments, net	0	0	(193)	0
Net amortization/accretion of discount/premium on investments	115	87	229	192
Transfers between investment types	0		0
Transfers into (out of) Level 3	(10,870)	0	0	0
Fair value, end of period	262,402	269,752	262,402	269,752
Unsecured debt investments | Net change in unrealized gain (loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in unrealized gain (loss) / realized gains (losses)	5,707	(19,295)	11,491	(24,942)
Unsecured debt investments | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in unrealized gain (loss) / realized gains (losses)	0	0	(23)	0
Preferred equity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	388,592	238,998	355,261	260,869
Purchases of investments, net	1,990	66,943	21,921	73,940
Payment-in-kind	10,094	5,845	21,072	9,431
Proceeds from investments, net	(1,589)	0	(1,590)	(33,693)
Net amortization/accretion of discount/premium on investments	236	177	471	966
Transfers between investment types	0		0
Transfers into (out of) Level 3	0	0	(48)	0
Fair value, end of period	400,625	296,766	400,625	296,766
Preferred equity investments | Net change in unrealized gain (loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in unrealized gain (loss) / realized gains (losses)	1,302	(15,197)	3,538	(19,229)
Preferred equity investments | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in unrealized gain (loss) / realized gains (losses)	0	0	0	4,482
Common equity investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	1,054,731	645,507	977,111	571,616
Purchases of investments, net	61,122	98,329	98,375	279,427
Payment-in-kind	179	155	342	308
Proceeds from investments, net	0	(25,753)	(12,400)	(128,731)
Net amortization/accretion of discount/premium on investments	0	0	0	0
Transfers between investment types	0		47,819
Transfers into (out of) Level 3	0	0	0	0
Fair value, end of period	1,131,331	730,326	1,131,331	730,326
Common equity investments | Net change in unrealized gain (loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in unrealized gain (loss) / realized gains (losses)	15,299	12,088	20,084	7,706
Common equity investments | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in unrealized gain (loss) / realized gains (losses)	0	0	0	0
Total
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	12,743,654	12,366,413	12,666,277	12,490,191
Purchases of investments, net	282,324	2,004,658	485,435	3,667,663
Payment-in-kind	62,127	39,640	112,840	69,211
Proceeds from investments, net	(680,457)	(1,992,664)	(853,886)	(3,639,397)
Net amortization/accretion of discount/premium on investments	9,645	11,202	18,376	23,405
Transfers between investment types	0		0
Transfers into (out of) Level 3	(50,437)	34,545	(86,904)	(67,279)
Fair value, end of period	12,383,853	12,303,511	12,383,853	12,303,511
Total | Net change in unrealized gain (loss)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in unrealized gain (loss) / realized gains (losses)	16,879	(160,234)	94,103	(244,941)
Total | Net realized gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Net change in unrealized gain (loss) / realized gains (losses)	$ 118	$ (49)	$ (52,388)	$ 4,658