Investments - Schedule of Composition of Investment Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2022	Mar. 31, 2022
Schedule of Investments [Line Items]
Investment cost	$ 2,607,174	$ 2,745,829
Fair Value	2,398,120	2,523,173
Level 1
Schedule of Investments [Line Items]
Fair Value	1,761	1,230
Level 2
Schedule of Investments [Line Items]
Fair Value	323
Level 3
Schedule of Investments [Line Items]
Fair Value	2,396,036	2,521,943
First Lien Secured Debt
Schedule of Investments [Line Items]
Investment cost	2,150,973	2,261,481
Fair Value	2,130,309	2,259,506
First Lien Secured Debt | Level 3
Schedule of Investments [Line Items]
Fair Value	2,130,309	2,259,506
Second Lien Secured Debt
Schedule of Investments [Line Items]
Investment cost	105,731	116,073
Fair Value	70,919	97,844
Second Lien Secured Debt | Level 3
Schedule of Investments [Line Items]
Fair Value	70,919	97,844
Unsecured Debt
Schedule of Investments [Line Items]
Investment cost	50
Fair Value	50
Unsecured Debt | Level 3
Schedule of Investments [Line Items]
Fair Value	50
Structured Products And Other
Schedule of Investments [Line Items]
Investment cost	16,998	16,998
Fair Value	9,413	10,038
Structured Products And Other | Level 3
Schedule of Investments [Line Items]
Fair Value	9,413	10,038
Preferred Equity
Schedule of Investments [Line Items]
Investment cost	40,530	41,152
Fair Value	35,557	30,162
Preferred Equity | Level 3
Schedule of Investments [Line Items]
Fair Value	35,557	30,162
Common Equity/Interests
Schedule of Investments [Line Items]
Investment cost	292,503	309,990
Fair Value	151,398	125,524
Common Equity/Interests | Level 1
Schedule of Investments [Line Items]
Fair Value	1,761	1,230
Common Equity/Interests | Level 2
Schedule of Investments [Line Items]
Fair Value	323
Common Equity/Interests | Level 3
Schedule of Investments [Line Items]
Fair Value	149,314	124,294
Warrants - Warrants
Schedule of Investments [Line Items]
Investment cost	389	135
Fair Value	474	99
Warrants - Warrants | Level 3
Schedule of Investments [Line Items]
Fair Value	$ 474	$ 99