Investment Securities (Proceeds and Gross Realized Gains And Losses from Sales of Securities Available for Sale) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Proceeds	$ 0	$ 1,064	[1]
Gross realized gains	$ 0	$ 1

[1] Adjusted for the retrospective adoption of ASU 2016-18. See Note 2.