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
	Mar. 31, 2023	Sep. 30, 2022	Mar. 31, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	$ 1,468,962		$ 704,651		$ 704,651
Net realized gain (loss) on investments	944		25
Net change in unrealized gain (loss) on investments	4,588		2,984
Purchases of investments and other adjustments to cost	104,141		81,148
Proceeds from principal payments and sales of investments	(34,777)		(22,272)
Balance at the end	1,543,858		766,536
Total net change in unrealized gain (loss) on investments attributable to Level 3 investments still held	5,633		3,164
Fair value, assets, transfers among Levels 1, 2 and 3		$ 0		$ 0	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	1,250,788		638,120		638,120
Net realized gain (loss) on investments	(5)		25
Net change in unrealized gain (loss) on investments	3,738		1,091
Purchases of investments and other adjustments to cost	97,250		56,166
Proceeds from principal payments and sales of investments	(31,623)		(22,238)
Reclassifications			(17,215)
Balance at the end	1,320,148		655,949
Unitranche Secured Loans
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	127,378		30,161		30,161
Net change in unrealized gain (loss) on investments	265		627
Purchases of investments and other adjustments to cost	890		3,345
Proceeds from principal payments and sales of investments	(153)		(34)
Reclassifications			17,215
Balance at the end	128,380		51,314
Junior Secured Loans
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	44,469		18,580		18,580
Net change in unrealized gain (loss) on investments	84		(255)
Purchases of investments and other adjustments to cost	2,119		19,078
Proceeds from principal payments and sales of investments	(2,039)
Balance at the end	44,633		37,403
Equity Securities
Reconciliation of beginning and ending balances for investments at fair value that use Level 3 inputs
Balance at the beginning	46,327		17,790		$ 17,790
Net realized gain (loss) on investments	949
Net change in unrealized gain (loss) on investments	501		1,521
Purchases of investments and other adjustments to cost	3,882		2,559
Proceeds from principal payments and sales of investments	(962)
Balance at the end	$ 50,697		$ 21,870