Borrowings - Weighted Average Cost and Amount of Advances (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Debt Disclosure [Abstract]
Weighted average interest rate, including cash flow hedges, at end of year	3.83%	2.02%	1.51%
Weighted daily average interest rate, including cash flow hedges, during the year	3.42%	1.66%	1.98%
Daily average of FHLB advances during the year	$ 2,916,849	$ 1,731,110	$ 2,234,027
Maximum amount of FHLB advances at any month end	3,425,000	2,125,000	2,700,000
Interest expense during the year (including swap interest income and expense)	$ 99,631	$ 28,729	$ 44,188