Borrowings - Adviser Revolver (Narrative) (Details) - USD ($)	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023	Jun. 13, 2025	Jun. 12, 2025
Debt Instrument [Line Items]
Long-term debt	$ 4,900,773,000	$ 4,599,430,000
Average stated interest rate	5.80%	6.20%	4.90%
Adviser Revolver
Debt Instrument [Line Items]
Interest rate	3.90%
Adviser Revolver | Line of Credit
Debt Instrument [Line Items]
Long-term debt	$ 39,200,000	$ 0
Other Short-Term Borrowings
Debt Instrument [Line Items]
Short-term debt, average outstanding amount	4,892,712,000	3,492,575,000	$ 3,069,412,000
Revolving Credit Facility | Adviser Revolver | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 300,000,000	$ 200,000,000		$ 300,000,000	$ 200,000,000
Interest rate	3.90%