Federal Income Tax Matters - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Investments, Owned, Federal Income Tax Note [Line Items]
Estimated capital loss carryforward available for use	$ 757
Amount of ordinary losses deferred	0	$ 0
Ordinary income	209,607	204,806	$ 189,204
Tax basis of investments, cost for income tax purposes	5,695,036
Unrealized gain on investment	104,381
Unrealized loss on investment	282,804
Realized and unrealized loss, investment and derivative, operating, tax (benefit) expense	(101)	1,229	$ 543
Accounts Payable and Accrued Liabilities
Investments, Owned, Federal Income Tax Note [Line Items]
Deferred income tax liabilities, net	1,090	$ 1,398
Investment Tax Credit Carryforward | Tax Year 2024
Investments, Owned, Federal Income Tax Note [Line Items]
Ordinary income	$ 56,886