Investment Securities (Narrative) (Details) $ in Millions	Sep. 30, 2017 USD ($)
Schedule of Investments [Line Items]
Gain on derivatives used to hedge purchase of investment securities	$ 58
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]
Amortized cost of debt securities of a single issuer that exceeeds 10 percent of shareholders equity	30,400
Fair value of debt securities of a single issuer that exceeeds 10 percent of shareholders equity	$ 30,400