Short and Long-Term Funding (Long-term Funding Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended
	Nov. 30, 2014	Sep. 30, 2018	Aug. 31, 2018	Dec. 31, 2017
Subordinated Borrowing [Line Items]
Carrying Value of Securities Purchased under Agreements to Resell and Deposits Paid for Securities Borrowed		$ 237,000
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		3,332,655		$ 3,184,168
Two Thousand Eighteen Senior Notes [Member] [Domain]
Subordinated Borrowing [Line Items]
New Senior Debt Issued			$ 300,000
Debt Instrument, Interest Rate, Stated Percentage			3.50%
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		1,371,059		$ 2,434,000
FLHB Advances, Putable [Domain]
Subordinated Borrowing [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		2,600,000
FHLB, Increase in Advances [Domain]
Subordinated Borrowing [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		$ 148,000