Borrowings - Adviser Revolver (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Other Short-Term Borrowings
Debt Instrument [Line Items]
Short-term debt, average outstanding amount	$ 3,069,412	$ 2,935,846	$ 2,184,010
Average stated interest rate	4.90%	3.00%	3.00%
Revolving Credit Facility | Adviser Revolver
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 100,000	$ 100,000
Revolving Credit Facility | Adviser Revolver | Short-term Applicable Federal Rate
Debt Instrument [Line Items]
Interest rate	5.00%