Short and Long-Term Funding (Long-term Funding Narrative) (Details) - USD ($) $ in Thousands			1 Months Ended	9 Months Ended
	Aug. 13, 2018	Nov. 13, 2014	Nov. 30, 2014	Sep. 30, 2019	Dec. 31, 2018
Long-Term Funding
FHLB advances				$ 2,877,727	$ 3,574,371
Carrying Value of Securities Purchased under Agreements to Resell and Deposits Paid for Securities Borrowed				164,000
Two Thousand Eighteen Senior Notes [Member] [Domain]
Long-Term Funding
Debt Instrument, Issuance Date Month, Year	2018-08
New Senior Debt Issued	$ 300,000
Debt Instrument, Maturity Date Month, Year	2021-08
Debt Instrument Call Date Earliest Month Year	2021-07
Debt Instrument, Interest Rate, Stated Percentage	3.50%
Two Thousand Fourteen Senior Notes [Member]
Long-Term Funding
Debt Instrument, Issuance Date Month, Year		2014-11
New Senior Debt Issued			$ 250,000
Debt Instrument, Maturity Date Month, Year		2019-11
Debt Instrument Call Date Earliest Month Year		2019-10
Debt Instrument, Interest Rate, Stated Percentage			2.75%
Debt Instrument, Called Date		Oct. 15, 2019
FHLBDecreaseinAdvances [Domain]
Long-Term Funding
FHLB advances					$ 697,000
FLHB Advances, Putable [Domain]
Long-Term Funding
FHLB advances				$ 2,400,000
FHLB Short Term Advances [Member]
Long-Term Funding
FHLB Put Contractual Maturity Weighted Average Life1				4 months 9 days
FHLB Long Term Advances [Member]
Long-Term Funding
FHLB Put Contractual Maturity Weighted Average Life1				5 years 5 months 1 day
Two Thousand Fourteen Subordinated Notes
Long-Term Funding
Debt Instrument, Issuance Date Month, Year		2014-11
Junior Subordinated Debentures Issued			$ 250,000
Debt Instrument, Term			10 years
Debt Instrument, Maturity Date Month, Year		2025-01
Debt Instrument Call Date Earliest Month Year		2024-10
Subordinated Borrowing, Interest Rate			4.25%