Accumulated Other Comprehensive Income/(Loss) - Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance			$ 256,515
Cumulative effect of changes in accounting principles						$ 88
Net change	$ 686	$ (1,287)	3,307	$ (2,394)
Ending balance	264,348	258,956	264,348	258,956
Unrealized gains/(losses) on investment securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	3,709	1,599	1,202	2,164
Cumulative effect of changes in accounting principles						896
Net change	479	(819)	2,986	(2,280)
Ending balance	4,188	780	4,188	780
Translation adjustments, net of hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(652)	(632)	(727)	(470)
Cumulative effect of changes in accounting principles						(277)
Net change	(165)	(31)	(90)	84
Ending balance	(817)	(663)	(817)	(663)
Fair value hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(73)	(162)	(161)	0
Cumulative effect of changes in accounting principles						(54)
Net change	(1)	34	87	(74)
Ending balance	(74)	(128)	(74)	(128)
Cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	126	(147)	(109)	76
Cumulative effect of changes in accounting principles						16
Net change	195	(88)	430	(327)
Ending balance	321	(235)	321	(235)
Defined benefit pension and OPEB plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(2,231)	(1,876)	(2,308)	(1,521)
Cumulative effect of changes in accounting principles						(414)
Net change	46	19	123	78
Ending balance	(2,185)	(1,857)	(2,185)	(1,857)
DVA on fair value option elected liabilities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	235	80	596	(368)
Cumulative effect of changes in accounting principles						(79)
Net change	132	(402)	(229)	125
Ending balance	367	(322)	367	(322)
Accumulated other comprehensive income/(loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	1,114	(1,138)	(1,507)	(119)
Cumulative effect of changes in accounting principles					$ 0	$ 88
Net change	686	(1,287)	3,307	(2,394)
Ending balance	$ 1,800	$ (2,425)	$ 1,800	$ (2,425)