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		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	$ 2,814,036	$ 1,543,858	$ 2,528,144	$ 1,468,962
Purchases of investments and other adjustments to cost	704,948	270,695	1,073,738	374,836
Proceeds from principal payments and sales of investments	(129,849)	(45,418)	(220,210)	(80,195)
Reclassifications	0	0	0	0
Transfers in (out) of Level 3			(133)
Balance at the end	$ 3,386,975	$ 1,764,742	$ 3,386,975	$ 1,764,742
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	$ (9,543)	$ (3,956)	$ (3,922)	$ 2,000
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	$ 220	$ (1,604)	$ 224	$ (660)
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	$ (2,380)	$ (2,789)	$ 5,212	$ 1,799
Senior secured loans
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	2,432,957	1,320,148	2,171,243	1,250,788
Purchases of investments and other adjustments to cost	607,868	254,104	948,674	351,354
Proceeds from principal payments and sales of investments	(123,094)	(41,663)	(198,881)	(73,286)
Reclassifications	(15,311)	(2,076)	(25,901)	(2,076)
Transfers in (out) of Level 3			0
Balance at the end	2,906,171	1,529,436	2,906,171	1,529,436
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	221	(1,604)	221	(1,609)
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	3,530	527	10,815	4,265
Unitranche secured loans
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	174,337	128,380	184,853	127,378
Purchases of investments and other adjustments to cost	1,235	993	3,673	1,883
Proceeds from principal payments and sales of investments	(6,666)	(285)	(20,347)	(438)
Reclassifications	0	0	0	0
Transfers in (out) of Level 3			0
Balance at the end	168,253	129,399	168,253	129,399
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	(653)	311	74	576
Junior secured loans
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	102,443	44,633	87,986	44,469
Purchases of investments and other adjustments to cost	47,108	1,931	62,167	4,050
Proceeds from principal payments and sales of investments	(73)	(3,362)	(347)	(5,401)
Reclassifications	12,277	2,076	8,615	2,076
Transfers in (out) of Level 3			0
Balance at the end	154,998	44,190	154,998	44,190
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	(6,757)	(1,088)	(3,423)	(1,004)
Equity securities
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	104,299	50,697	84,062	46,327
Purchases of investments and other adjustments to cost	48,737	13,667	59,224	17,549
Proceeds from principal payments and sales of investments	(16)	(108)	(635)	(1,070)
Reclassifications	3,034	0	17,286	0
Transfers in (out) of Level 3			(133)
Balance at the end	157,553	61,717	157,553	61,717
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	(1)	0	3	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	$ 1,500	$ (2,539)	$ (2,254)	$ (2,038)