Borrowings - 2026 Notes (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Oct. 13, 2021	Aug. 24, 2021	Feb. 24, 2021
Debt Instrument [Line Items]
Debt	$ 3,133,332	$ 3,093,603
US Treasury (UST) Interest Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	0.25%
2026 Notes
Debt Instrument [Line Items]
Debt instrument, face amount			$ 200,000		$ 400,000
Debt	$ 598,461	$ 600,000
Interest rate				2.50%
Debt instrument, redemption price, percentage	100.00%
2026 Notes | US Treasury (UST) Interest Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	0.30%