Short and Long-Term Funding (Long-term Funding Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Nov. 30, 2014	Mar. 31, 2019	Dec. 31, 2018	Aug. 31, 2018
Long-Term Funding
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		$ 2,944,769	$ 3,574,371
Subordinated Borrowing [Line Items]
Carrying Value of Securities Purchased under Agreements to Resell and Deposits Paid for Securities Borrowed		171,000
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%
Two Thousand Eighteen Senior Notes [Member] [Domain]
Long-Term Funding
New Senior Debt Issued				$ 300,000
Debt Instrument, Interest Rate, Stated Percentage				3.50%
Two Thousand Fourteen Senior Notes [Member]
Long-Term Funding
New Senior Debt Issued	$ 250,000
Debt Instrument, Interest Rate, Stated Percentage	2.75%
FHLBDecreaseinAdvances [Domain]
Long-Term Funding
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		630,000
FLHB Advances, Putable [Domain]
Long-Term Funding
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		$ 2,800,000
FHLB Short Term Advances [Member]
Long-Term Funding
FHLB Put Contractual Maturity Weighted Average Life1		8 months 8 days
FHLB Long Term Advances [Member]
Long-Term Funding
FHLB Put Contractual Maturity Weighted Average Life1		7 years 29 days