Investment Securities (Narrative) (Details) $ in Millions	Mar. 31, 2019 USD ($)
Schedule of Investments [Line Items]
Investment securities, net unsettled investment purchases	$ 623
Gain on derivatives used to hedge purchase of investment securities	25
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	37,100
Fair value of debt securities of a single issuer that exceeeds 10 percent of shareholders equity	$ 37,000