Investment Securities (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Period Increase (Decrease)
Gain on derivatives used to hedge purchase of investment securities	54
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	1,100
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Debt Securities
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Investments [Line Items]
Amortized cost of debt securities of a single issuer that exceeeds 10 percent of shareholders equity	31,000
Fair value of debt securities of a single issuer that exceeeds 10 percent of shareholders equity	$ 30,800