Loans - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans covered by a FDIC loss sharing agreement	$ 0		$ 0		$ 31,891
Loan held for sale	12,840		12,840		7,351
Loans at fair value under the fair value option	735,363		735,363		812,991
Unamortized discount on acquired loans	33,800		33,800		13,900
Loan in process	500		500		1,600
Financing receivable, after allowance for credit loss	10,165,841		10,165,841		9,635,989
Principal balances of residential real estate loans sold	50,600	$ 69,800	226,000	$ 170,400
Loans greater than 90 days past due and still accruing interest	0		0		11,200
Valuation adjustments made to repossessed properties	2,000	100	6,800	2,000
Specific reserves included in the allowance for loan losses for TDRs			7,900		10,300
Troubled debt restructuring, commitments to lend additional funds					200
Transfers out of troubled debt restructuring status	0	$ 0	300	$ 0
Loans Guaranteed by US Government Authorities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, after allowance for credit loss	$ 865,800		$ 865,800		$ 154,200