Investment Securities (Narrative) (Details) $ in Millions	Mar. 31, 2018 USD ($)
Schedule of Investments [Line Items]
Gain on derivatives used to hedge purchase of investment securities	$ 50
Other Than Temporary Impairment Credit Losses Recognized In Earnings Credit Losses On Debt Securities Held	1,100
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]
Schedule of Investments [Line Items]
Debt Securities Amortized Cost Amount of Debt Securities Exceeds 10 Percent of Shareholders Equity	32,700
Fair value of debt securities of a single issuer that exceeeds 10 percent of shareholders equity	$ 31,900