Loans - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans covered by FDIC loss share agreements	$ 68,553		$ 73,272
Loan held for sale	9,086		12,918
Loans and written loan commitments at fair value under the fair value option	1,078,465		1,131,111
Unamortized discount on acquired loans	9,200		8,600
Loans in process	4,400		4,700
Loans guaranteed by U.S. Government Agencies	7,556,647		7,431,936
Principal balances of residential real estate loans sold	91,700	$ 58,200
Loans greater than 90 days past due and still accruing interest	100		2,000
Valuation adjustments made to repossessed properties	400	0
Specific reserves included in the allowance for loan losses for TDRs	7,900		9,300
Troubled debt restructuring, commitments to lend additional funds	2,100		900
Transfers out of troubled debt restructuring status	0	$ 4,300
Loans Guaranteed by US Government Authorities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans guaranteed by U.S. Government Agencies	$ 148,500		$ 120,000