Short and Long-Term Funding (Long-term Funding Narrative) (Details) - USD ($) $ in Millions	1 Months Ended
	Nov. 30, 2014	Mar. 31, 2018	Dec. 31, 2017
Subordinated Borrowing [Line Items]
Carrying Value of Securities Purchased under Agreements to Resell and Deposits Paid for Securities Borrowed		$ 288
Federal Home Loan Bank, Advances, Weighted Average Interest Rate		1.50%	1.26%
Two Thousand Fourteen Senior Notes
Subordinated Borrowing [Line Items]
New Senior Debt Issued	$ 250
Debt Instrument, Interest Rate, Stated Percentage	2.75%
Two Thousand Fourteen Subordinated Notes
Subordinated Borrowing [Line Items]
Junior Subordinated Debentures Issued	$ 250
Debt Instrument, Term	10 years
Subordinated Borrowing, Interest Rate	4.25%