Accumulated Other Comprehensive Income/(Loss) - Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018	Jan. 01, 2017	Jan. 01, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 255,693	$ 254,190
Cumulative effect of change in accounting principles						$ 154
Net change	(1,476)	1,056	$ (1,521)
Ending balance	256,515	255,693	254,190
Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				$ 261
Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				115
Reclassification of certain tax effects from AOCI
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				(288)
Unrealized gains/(losses) on investment securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	2,164	1,524	2,629
Cumulative effect of change in accounting principles						0
Net change	(1,858)	640	(1,105)
Ending balance	1,202	2,164	1,524
Unrealized gains/(losses) on investment securities | Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				261
Unrealized gains/(losses) on investment securities | Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				169
Unrealized gains/(losses) on investment securities | Reclassification of certain tax effects from AOCI
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				466
Translation adjustments, net of hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(470)	(164)	(162)
Cumulative effect of change in accounting principles						0
Net change	20	(306)	(2)
Ending balance	(727)	(470)	(164)
Translation adjustments, net of hedges | Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				0
Translation adjustments, net of hedges | Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				0
Translation adjustments, net of hedges | Reclassification of certain tax effects from AOCI
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				(277)
Fair value hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	0
Net change	(107)
Ending balance	(161)	0
Fair value hedges | Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				0
Fair value hedges | Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				(54)
Fair value hedges | Reclassification of certain tax effects from AOCI
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				0
Cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	76	(100)	(44)
Cumulative effect of change in accounting principles						0
Net change		176	(56)
Ending balance		76	(100)
Cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Net change	(201)
Ending balance	(109)
Cash flow hedges | Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				0
Cash flow hedges | Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				0
Cash flow hedges | Reclassification of certain tax effects from AOCI
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				16
Defined benefit pension and OPEB plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(1,521)	(2,259)	(2,231)
Cumulative effect of change in accounting principles						0
Net change	(373)	738	(28)
Ending balance	(2,308)	(1,521)	(2,259)
Defined benefit pension and OPEB plans | Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				0
Defined benefit pension and OPEB plans | Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				0
Defined benefit pension and OPEB plans | Reclassification of certain tax effects from AOCI
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				(414)
DVA on fair value option elected liabilities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(368)	(176)	0
Cumulative effect of change in accounting principles						154
Net change	1,043	(192)	(330)
Ending balance	596	(368)	(176)
DVA on fair value option elected liabilities | Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				0
DVA on fair value option elected liabilities | Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				0
DVA on fair value option elected liabilities | Reclassification of certain tax effects from AOCI
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				(79)
Accumulated other comprehensive income/(loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(119)	(1,175)	192
Cumulative effect of change in accounting principles				88	$ 0	$ 154
Net change	(1,476)	1,056	(1,521)
Ending balance	$ (1,507)	$ (119)	$ (1,175)
Accumulated other comprehensive income/(loss) | Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				261
Accumulated other comprehensive income/(loss) | Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				115
Accumulated other comprehensive income/(loss) | Reclassification of certain tax effects from AOCI
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of change in accounting principles				$ (288)