Fair Value Measurements - Reconciliation of the Beginning and Ending Balances for Investments at Fair Value that use Level 3 Inputs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	$ 3,386,975	$ 1,764,742	$ 2,528,144	$ 1,468,962
Purchases of investments and other adjustments to cost	442,466	235,523	1,516,204	610,359
Proceeds from principal payments and sales of investments	(180,175)	(23,534)	(400,385)	(103,729)
Reclassifications	0	0	0	0
Transfers in (out) of Level 3			(133)
Balance at the end	$ 3,660,054	$ 1,974,109	$ 3,660,054	$ 1,974,109
Fair value, asset, recurring basis, still held, unrealized gain (loss), statement of income or comprehensive income [extensible enumeration]	Unrealized gain (loss) of investments	Unrealized gain (loss) of investments	Unrealized gain (loss) of investments	Unrealized gain (loss) of investments
Total net change in unrealized gain (loss) on investments attributable to Level 3 investments still held	$ 16,806	$ (2,913)	$ 10,096	$ (949)
Net realized gain (loss) on investments
Fair Value Measurements
Fair value, asset, recurring basis, unobservable input reconciliation, gain (loss), statement of income or comprehensive income [extensible enumeration]	Non-controlled/non-affiliate company investments	Non-controlled/non-affiliate company investments	Non-controlled/non-affiliate company investments	Non-controlled/non-affiliate company investments
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Gains (losses) included in earnings	$ 495	$ 38	$ 719	$ (622)
Net change in unrealized gain (loss) on investments
Fair Value Measurements
Fair value, asset, recurring basis, unobservable input reconciliation, gain (loss), statement of income or comprehensive income [extensible enumeration]	Unrealized gain (loss) of investments	Unrealized gain (loss) of investments	Unrealized gain (loss) of investments	Unrealized gain (loss) of investments
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Gains (losses) included in earnings	$ 10,293	$ (2,660)	$ 15,505	$ (861)
Senior secured loans
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	2,906,171	1,529,436	2,171,243	1,250,788
Purchases of investments and other adjustments to cost	422,547	228,395	1,371,221	579,749
Proceeds from principal payments and sales of investments	(173,211)	(21,252)	(372,092)	(94,538)
Reclassifications	(3,808)	(9,049)	(29,709)	(11,125)
Transfers in (out) of Level 3			0
Balance at the end	3,151,328	1,726,087	3,151,328	1,726,087
Senior secured loans | Net realized gain (loss) on investments
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Gains (losses) included in earnings	342	38	563	(1,571)
Senior secured loans | Net change in unrealized gain (loss) on investments
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Gains (losses) included in earnings	(713)	(1,481)	10,102	2,784
Unitranche secured loans
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	168,253	129,399	184,853	127,378
Purchases of investments and other adjustments to cost	3,425	1,289	7,098	3,172
Proceeds from principal payments and sales of investments	(5,956)	(1,028)	(26,303)	(1,466)
Reclassifications	0	0	0	0
Transfers in (out) of Level 3			0
Balance at the end	168,208	129,840	168,208	129,840
Unitranche secured loans | Net realized gain (loss) on investments
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Gains (losses) included in earnings	0	0	0	0
Unitranche secured loans | Net change in unrealized gain (loss) on investments
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Gains (losses) included in earnings	2,486	180	2,560	756
Junior secured loans
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	154,998	44,190	87,986	44,469
Purchases of investments and other adjustments to cost	3,037	2,662	65,204	6,712
Proceeds from principal payments and sales of investments	(240)	(1,254)	(587)	(6,655)
Reclassifications	405	7,939	9,020	10,015
Transfers in (out) of Level 3			0
Balance at the end	160,750	51,521	160,750	51,521
Junior secured loans | Net realized gain (loss) on investments
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Gains (losses) included in earnings	0	0	0	0
Junior secured loans | Net change in unrealized gain (loss) on investments
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Gains (losses) included in earnings	2,550	(2,016)	(873)	(3,020)
Equity securities
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	157,553	61,717	84,062	46,327
Purchases of investments and other adjustments to cost	13,457	3,177	72,681	20,726
Proceeds from principal payments and sales of investments	(768)	0	(1,403)	(1,070)
Reclassifications	3,403	1,110	20,689	1,110
Transfers in (out) of Level 3			(133)
Balance at the end	179,768	66,661	179,768	66,661
Equity securities | Net realized gain (loss) on investments
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Gains (losses) included in earnings	153	0	156	949
Equity securities | Net change in unrealized gain (loss) on investments
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Gains (losses) included in earnings	$ 5,970	$ 657	$ 3,716	$ (1,381)