Borrowings (Federal Home Loan Advances) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Federal Home Loan Bank, Advances, Maturity, Rolling Year [Abstract]
FHLB advances maturing within 1 year, Total Outstanding	$ 880,500	$ 2,130,500
FHLB advances maturing after 1 but within 2 years, Total Outstanding	133,731	200,000
FHLB advances maturing after 2 but within 3 years, Total Outstanding	259,295	128,026
FHLB advances maturing after 3 but within 4 years, Total Outstanding	75,000	175,000
FHLB advances maturing after 4 but within 5 years, Total Outstanding	150,061	200,000
FHLB advances maturing after 5 years, Total Outstanding	9,091	9,370
Federal Home Loan Bank, Advances	1,507,678	2,842,896
Premiums on advances	3	12
Federal Home Loan Bank advances	$ 1,507,681	$ 2,842,908
Federal Home Loan Bank, Advances, Weighted Average Interest Rate [Abstract]
FHLB advances maturing within 1 year, Weighted Average Contractual Coupon Rate (as a percent)	1.28%	0.71%
FHLB advances maturing after 1 but within 2 years, Weighted Average Contractual Coupon Rate (as a percent)	1.34%	1.36%
FHLB advances maturing after 2 but within 3 years, Weighted Average Contractual Coupon Rate (as a percent)	1.79%	1.73%
FHLB advances maturing after 3 but within 4 years, Weighted Average Contractual Coupon Rate (as a percent)	1.51%	1.77%
FHLB advances maturing after 4 but within 5 years, Weighted Average Contractual Coupon Rate (as a percent)	2.23%	1.81%
FHLB advances maturing after 5 years, Weighted Average Contractual Coupon Rate (as a percent)	2.61%	2.59%
Federal Home Loan Bank, Advances, Weighted Average Contractual Coupon Rate (as a percent)	1.49%	0.95%
Pledged Financial Instruments, Not Separately Reported, Loans Receivable, for Federal Home Loan Bank Debt	$ 6,388,102	$ 5,967,318
Remaining borrowing capacity	$ 2,668,964	$ 1,192,758