Loans - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans covered by a FDIC loss sharing agreement	$ 29,691			$ 29,691		$ 31,891
Loan held for sale	4,342			4,342		7,351
Loans and written loan commitments at fair value under the fair value option	792,117			792,117		812,991
Unamortized discount on acquired loans	13,700			13,700		13,900
Loans in process	400			400		1,600
Loans guaranteed by U.S. Government Agencies	8,772,155			8,772,155		8,752,832
Principal balances of residential real estate loans sold	68,400		$ 46,800	175,400	$ 100,700
Loans greater than 90 days past due and still accruing interest	2,300			2,300		11,200
Valuation adjustments made to repossessed properties	4,800		100	4,800	2,000
Specific reserves included in the allowance for loan losses for TDRs		$ 10,300		6,900
Troubled debt restructuring, commitments to lend additional funds	100			100		200
Transfers out of troubled debt restructuring status	300		$ 0	300	$ 0
Loans Guaranteed by US Government Authorities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans guaranteed by U.S. Government Agencies	$ 146,200			$ 146,200		$ 154,200