Fair Value Measurements - Reconciliation of 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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	$ 480,614	$ 497,940	$ 505,237	$ 519,527
Net realized gain (loss) on investments	6	(1,667)	(39,020)	(1,750)
Net change in unrealized gain (loss) on investments	(4,387)	(5,068)	22,062	(17,106)
Purchases of investments and other adjustments to cost	15,298	52,915	60,270	99,105
Proceeds from principal payments and sales of investments	(6,745)	(73,027)	(63,763)	(128,683)
Balance at the end	484,786	471,093	484,786	471,093
Total net change in unrealized gain (loss) on investments attributable to Level 3 investments still held	(5,664)	(6,132)	$ (9,596)	(16,041)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]			Derivative and Foreign Currency Transaction Price Change, Realized Gain (Loss), Operating, before Tax
Fair value, assets, transfers among Levels 1, 2 and 3			$ 0	0
Senior Secured Loans
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	415,873	415,244	434,023	423,700
Net realized gain (loss) on investments		(1,672)	(9,787)	(1,693)
Net change in unrealized gain (loss) on investments	(3,554)	(3,788)	(2,734)	(8,838)
Purchases of investments and other adjustments to cost	13,259	52,370	54,107	88,950
Proceeds from principal payments and sales of investments	(5,971)	(59,925)	(53,234)	(99,890)
Reclassifications	(3,291)		(6,059)
Balance at the end	416,316	402,229	416,316	402,229
Unitranche Secured Loans
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	13,400	34,114	20,633	51,494
Net realized gain (loss) on investments			(24,768)	(94)
Net change in unrealized gain (loss) on investments	(43)	10	24,746	(5,146)
Purchases of investments and other adjustments to cost	210	15	295	3,253
Proceeds from principal payments and sales of investments	(19)	(13,096)	(7,358)	(28,464)
Balance at the end	13,548	21,043	13,548	21,043
Junior Secured Loans
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	25,307	19,745	22,193	14,364
Net realized gain (loss) on investments		(1)		(1)
Net change in unrealized gain (loss) on investments	(690)	1,314	(1,743)	992
Purchases of investments and other adjustments to cost	1,746	368	4,564	6,071
Proceeds from principal payments and sales of investments	(749)		(2,168)
Reclassifications	1,811		4,579
Balance at the end	27,425	21,426	27,425	21,426
Equity Securities
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	26,034	28,837	28,388	29,969
Net realized gain (loss) on investments	6	6	(4,465)	38
Net change in unrealized gain (loss) on investments	(100)	(2,604)	1,793	(4,114)
Purchases of investments and other adjustments to cost	83	162	1,304	831
Proceeds from principal payments and sales of investments	(6)	(6)	(1,003)	(329)
Reclassifications	1,480		1,480
Balance at the end	$ 27,497	$ 26,395	$ 27,497	$ 26,395