Fair Value Measurements - Reconciliation of Beginning and Ending Balances for Investments at Fair Value that Use Level 3 Inputs (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2024 USD ($) investment	Mar. 31, 2023 USD ($)
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	$ 455,007	$ 505,237
Purchases of investments and other adjustments to cost	26,708	24,946
Proceeds from principal payments and sales of investments	(12,087)	(30,404)
Reclassifications	0
Transfers in (out) of Level 3	(335)
Balance at the end	466,879	496,038
Total net change in unrealized gain (loss) on investments attributable to Level 3 investments still held	$ (2,451)	(3,645)
Number of investments transferred from level 3 | investment	1,000
Net realized gain (loss) on investments
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Net realized gain (loss) on investments	$ 4	706
Net change in unrealized gain (loss) on investments
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Net realized gain (loss) on investments	(2,418)	(4,447)
Senior secured loans
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	388,882	434,023
Purchases of investments and other adjustments to cost	23,576	23,577
Proceeds from principal payments and sales of investments	(4,038)	(21,451)
Reclassifications	(2,712)
Transfers in (out) of Level 3	0
Balance at the end	404,051	431,880
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
Net realized gain (loss) on investments	0	0
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
Net realized gain (loss) on investments	(1,657)	(4,269)
Unitranche secured loans
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	13,877	20,633
Purchases of investments and other adjustments to cost	100	45
Proceeds from principal payments and sales of investments	(7,785)	(7,320)
Reclassifications	0
Transfers in (out) of Level 3	0
Balance at the end	6,091	13,361
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
Net realized gain (loss) on investments	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
Net realized gain (loss) on investments	(101)	3
Junior secured loans
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	26,594	22,193
Purchases of investments and other adjustments to cost	1,087	569
Proceeds from principal payments and sales of investments	(260)	(927)
Reclassifications	(1,407)
Transfers in (out) of Level 3	0
Balance at the end	27,427	21,978
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
Net realized gain (loss) on investments	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
Net realized gain (loss) on investments	1,413	143
Equity securities
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	25,654	28,388
Purchases of investments and other adjustments to cost	1,945	755
Proceeds from principal payments and sales of investments	(4)	(706)
Reclassifications	4,119
Transfers in (out) of Level 3	(335)
Balance at the end	29,310	28,819
Equity securities | Net realized gain (loss) on investments
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Net realized gain (loss) on investments	4	706
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
Net realized gain (loss) on investments	$ (2,073)	$ (324)