Federal Income Tax Matters - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Investments, Owned, Federal Income Tax Note [Line Items]
Estimated capital loss carryforward available for use	$ (53,950,000)
Amount of ordinary losses deferred	(6,981,000)	$ 0	$ 0
Tax basis of investments, cost for income tax purposes	8,728,590,000
Unrealized gain on investment	314,813,000
Unrealized loss on investment	274,014,000
Net tax benefit for taxable subsidiaries	563,000	549,000	$ 101,000
Accounts Payable and Accrued Liabilities
Investments, Owned, Federal Income Tax Note [Line Items]
Deferred income tax liabilities, net	$ 217,000	$ 780,000