Significant Accounting Policies and Recent Accounting Updates - Narrative (Details) - USD ($) $ in Thousands			12 Months Ended
	May 28, 2021	Jan. 01, 2020	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income, operating, accretion of discounts			$ 24,679	$ 21,399	$ 16,437
Amortization of deferred loan origination fees, net			27,023	34,215	13,072
PIK interest			21,506	16,092	10,956
Investment company, premium prepayment fees			3,040	3,200	1,184
Interest and fee income			354,032	301,687	312,933
Return of Capital			0	0	6,625
Fair value non-accrual loan			65,125	46,104
Amortization of debt premium			15,632	30,793	39,920
Total assets, at fair value:			5,515,897	4,935,919
Stock repurchase program, authorized amount			150,000
Offering costs, partnership interests	$ 250,000
Maximum percentage of gross sales, commission received, percentage	1.25%
Deferred debt issuance cost			17,211	17,850
Amortization of deferred debt issuance costs			7,337	10,203	3,534
Non-Senior Loan Fund
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Dividend income			684	1,713	291
Return of Capital			1,146	542	697
Senior Loan Fund
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Dividend income			0	0	1,905
Return of Capital			$ 0	$ 0	$ 4,375
Asset acquisition, consideration transferred		$ 140,124
Deconsolidation, gain, amount		123,113
Total assets, at fair value:		136,088
Investment income, net, amortization of premium		4,036
Senior Loan Fund | RGA and Aurora
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cash paid of minority interest		$ 17,011