FHLB Advances and Other Borrowings - Weighted Average Cost and Amount of Advances (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Federal Home Loan Banks [Abstract]
Weighted average interest rate, including cash flow hedges, at end of year	1.51%	1.79%	2.49%
Weighted daily average interest rate, including cash flow hedges, during the year	1.98%	2.03%	2.69%
Daily average of FHLB advances during the year	$ 2,234,027	$ 2,532,596	$ 2,533,890
Maximum amount of FHLB advances at any month end	2,700,000	3,050,000	2,665,000
Interest expense during the year (including swap interest income and expense)	$ 44,188	$ 51,445	$ 68,190