Federal Income Tax Matters - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Investments, Owned, Federal Income Tax Note [Line Items]
Deferred tax assets, capital loss carryforwards	$ 0	$ 2,270,000
Ordinary income	204,806,000	189,204,000	$ 190,874,000
Long-term capital gains	0	5,648,000	4,691,000
Tax basis of investments, cost for income tax purposes	5,683,319,000
Unrealized gain on investment	84,731,000
Unrealized loss on investment	(321,694,000)
Realized and unrealized gain (loss), investment and derivative, operating, tax expense (benefit)	1,229,000	543,000	0
Investment Tax Credit Carryforward | Tax Year 2023
Investments, Owned, Federal Income Tax Note [Line Items]
Ordinary income	39,466,000
Long-term capital gains	28,784,000
Accounts Payable and Accrued Liabilities
Investments, Owned, Federal Income Tax Note [Line Items]
Deferred income tax liabilities, net	$ 1,398,000	$ 543,000
Short-term Capital Loss
Investments, Owned, Federal Income Tax Note [Line Items]
Deferred tax assets, capital loss carryforwards			81,000
Long-term Capital Loss
Investments, Owned, Federal Income Tax Note [Line Items]
Deferred tax assets, capital loss carryforwards			$ 11,886,000