Allowance for Losses on Financial Assets (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for Losses on Financial Assets [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 18,075		$ 8,300	$ (410)	$ (1,713)
Allowance for Credit Loss	(802)	$ (337)
Retained Earnings [Member]
Allowance for Losses on Financial Assets [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ (38,610)		$ (50,644)	$ (55,941)	$ (49,618)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member]
Allowance for Losses on Financial Assets [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		162
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member] | Retained Earnings [Member]
Allowance for Losses on Financial Assets [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		$ 162