Loans - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans covered by FDIC loss share agreement	$ 0	$ 31,891
Loan held for sale	12,371	7,351
Loans and written loan commitments at fair value under the fair value option	655,185	812,991
Unamortized discount on acquired loans	29,000	13,900
Loans in process	(1,800)	(1,600)
Guaranteed loans	8,573,510	8,752,832
Principal balances of residential real estate loans sold	496,900	288,100
Loans greater than 90 days past due and still accruing interest	0	11,200
Valuation adjustments made to repossessed properties	10,800	2,300
Specific reserves included in the allowance for loan losses for TDRs	11,000	10,300
Troubled debt restructuring, commitments to lend additional funds to borrowers	0	200
Transfers out of troubled debt restructuring status	300	0	$ 800
Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and written loan commitments at fair value under the fair value option	655,200	813,000
Loans Guaranteed by US Government Authorities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Guaranteed loans	$ 869,300	$ 154,200