Deposits and Borrowed Funds (FHLB Advances) (Details) - USD ($) $ in Thousands		Sep. 30, 2016	Sep. 30, 2015
Debt [Line Items]
Fixed-rate FHLB advances		$ 2,375,000	$ 2,575,000
Deferred prepayment penalty		(2,611)	(4,479)
FHLB advances, outstanding balance		$ 2,372,389	$ 3,270,521
Weighted Average [Member]
Debt [Line Items]
Weighted average contractual interest rate on FHLB advances		2.17%	2.09%
Weighted average effective interest rate on FHLB advances	[1]	2.24%	2.24%
FHLB Advances [Member]
Debt [Line Items]
FHLB advances, outstanding balance		$ 2,372,389	$ 2,570,521

[1] The effective interest rate includes the net impact of deferred amounts related to certain FHLB advances.