Investment Securities (Narrative) (Details) $ in Millions	Sep. 30, 2018 USD ($)
Schedule of Investments [Line Items]
Gain on derivatives used to hedge purchase of investment securities	$ 27
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	36,500
Fair value of debt securities of a single issuer that exceeeds 10 percent of shareholders equity	$ 35,300