Short and Long-Term Funding (Long-term Funding Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended
	Nov. 30, 2014	Jun. 30, 2018	Dec. 31, 2017
Subordinated Borrowing [Line Items]
Carrying Value of Securities Purchased under Agreements to Resell and Deposits Paid for Securities Borrowed		$ 279,000
Federal Home Loan Bank, Advances, Weighted Average Interest Rate		1.97%	1.26%
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		$ 4,797,857	$ 3,184,168
FHLB Advances, Weighted Average Life		1 year 4 months 24 days
FHLB Put Contractual Maturity Weighted Average Life		6 years 7 months 6 days
Two Thousand Fourteen Senior Notes
Subordinated Borrowing [Line Items]
New Senior Debt Issued	$ 250,000
Debt Instrument, Interest Rate, Stated Percentage	2.75%
Two Thousand Fourteen Subordinated Notes
Subordinated Borrowing [Line Items]
Junior Subordinated Debentures Issued	$ 250,000
Debt Instrument, Term	10 years
Subordinated Borrowing, Interest Rate	4.25%
FHLB Maturity within One Year [Domain]
Subordinated Borrowing [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		$ 2,632,613	$ 2,434,000
FHLB, Increase in Advances [Domain]
Subordinated Borrowing [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		$ 1,600,000