Short and Long-term Funding Short and Long-term Funding (Details) - USD ($) $ in Thousands	1 Months Ended
	Nov. 30, 2014	Dec. 31, 2018	Aug. 31, 2018	Dec. 31, 2017
Subordinated Borrowing [Line Items]
Collateral for repurchase agreements		$ 194,000
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		3,574,371		$ 3,184,168
FHLB, Increase in Advances [Domain]
Subordinated Borrowing [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		390,000
FLHB Advances, Putable [Domain]
Subordinated Borrowing [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances		2,600,000
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 [Member]
Subordinated Borrowing [Line Items]
New Senior Debt Issued	$ 250,000
Debt Instrument, Interest Rate, Stated Percentage	2.75%
Two Thousand Fourteen Subordinated Notes [Member]
Subordinated Borrowing [Line Items]
Junior Subordinated Debentures Issued	$ 250,000
Debt Instrument, Term	10 years
Subordinated Borrowing, Interest Rate	4.25%
Maximum
Subordinated Borrowing [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged		$ 7,100,000