Borrowings - Adviser Revolver (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Jun. 11, 2024	Sep. 30, 2023
Debt Instrument [Line Items]
Long-term debt	$ 4,315,465		$ 4,315,465			$ 3,117,719
Average stated interest rate	6.50%	5.10%	5.90%	4.80%
Adviser Revolver | Line of Credit
Debt Instrument [Line Items]
Long-term debt	$ 0		$ 0			0
Other Short-Term Borrowings
Debt Instrument [Line Items]
Short-term debt, average outstanding amount	3,351,951	$ 3,097,509	3,211,972	$ 3,071,718
Revolving Credit Facility | Adviser Revolver | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 200,000		$ 200,000		$ 200,000	$ 100,000
Revolving Credit Facility | Adviser Revolver | Short-term Applicable Federal Rate | Line of Credit
Debt Instrument [Line Items]
Interest rate	5.00%		5.00%