Allowance for Loan and Lease Losses - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and written loan commitments at fair value under the fair value option	$ 1,049,548		$ 1,049,548			$ 1,131,111
Loan held for sale	11,504		11,504			12,918
Loans guaranteed by U.S. Government Agencies	7,575,529		7,575,529			7,431,936
Allowance for loan leases	64,214	$ 64,243	64,214	$ 64,243	$ 62,685	64,642	$ 61,917	$ 57,200
Net reversal of provision	936	1,105	2,550	3,300
Unfunded loan commitment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reserve for unfunded loan commitments	500		500			500
(Impairment) improvement of ASC 310-30 loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans guaranteed by U.S. Government Agencies	106,763		106,763			129,531
Allowance for loan leases	658		658			1,679
Net reversal of provision	100	$ 400	1,000	$ 600
Loans Guaranteed by US Government Authorities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans guaranteed by U.S. Government Agencies	$ 166,100		$ 166,100			$ 120,000