Significant Accounting Policies and Recent Accounting Updates - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	May 28, 2021	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022	Sep. 30, 2022	Aug. 05, 2022
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income, operating, accretion of discounts		$ 4,663	$ 4,048	$ 10,691	$ 11,783
Amortization of deferred loan origination fees, net		4,762	4,804	11,386	16,016
Payment-in-kind interest income		9,164	4,613	17,683	8,943
Payment-in-kind interest capitalized		9,478	5,124	17,255	8,597
Investment company, premium prepayment fees		0	103	79	795
Interest and fee income		127,648	82,033	241,181	162,023
Non-cash preferred dividends		3,595	0	7,044	0
Fair value non-accrual loan		88,581		88,581		$ 65,125
Amortization of debt premium		1,652	2,940	3,787	10,035
Excise tax expense (reduction), net		232	0	2,432	0
Offering costs, partnership interests	$ 250,000
Maximum percentage of gross sales, commission received, percentage	1.25%
Deferred debt issuance cost		19,154		19,154		$ 17,211
Amortization of deferred debt issuance costs		1,677	1,776	3,425	3,443
Common Stock
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stock repurchase program, authorized amount							$ 150,000
Non-Senior Loan Fund
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Dividend income		311	3	995	320
Return of Capital		$ 83	$ 596	$ 83	$ 726