Loans - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans covered by FDIC loss share agreements	$ 73,272	$ 97,030
Loans held for sale	12,918	9,867
Loans and written loan commitments at fair value under the fair value option	1,131,111	1,118,687
Unamortized discount on acquired loans	8,600	7,500
Loans in process	4,700	(700)
Guaranteed loans	7,431,936	6,098,475
Principal balances of residential real estate loans sold	291,200	281,600
Loans greater than 90 days past due and still accruing interest	2,000	100
Valuation adjustments made to repossessed properties	1,800	5,900
Specific reserves included in the allowance for loan losses for TDRs	9,300	3,600
Troubled debt restructuring, commitments to lend additional funds to borrowers	900	2,300
Transfers out of troubled debt restructuring status	22,300	14,500	$ 12,900
Loans Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and written loan commitments at fair value under the fair value option	1,130,000	1,120,000
Loans Guaranteed by US Government Authorities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Guaranteed loans	$ 120,000	$ 105,000