Accumulated Other Comprehensive Income/(Loss) - Rollforward (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 256,515
Net change	949	$ (1,032)
Ending balance	259,837	256,201
Unrealized gains/(losses) on investment securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	1,202	2,164
Cumulative effect of changes in accounting principles				$ 896
Net change	1,414	(1,234)
Ending balance	2,616	1,826
Translation adjustments, net of hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(727)	(470)
Cumulative effect of changes in accounting principles				(277)
Net change	(24)	27
Ending balance	(751)	(720)
Fair value hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(161)	0
Cumulative effect of changes in accounting principles				(54)
Net change	2	(40)
Ending balance	(159)	(94)
Cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(109)	76
Cumulative effect of changes in accounting principles				16
Net change	138	(73)
Ending balance	29	19
Defined benefit pension and OPEB plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(2,308)	(1,521)
Cumulative effect of changes in accounting principles				(414)
Net change	36	21
Ending balance	(2,272)	(1,914)
DVA on fair value option elected liabilities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	596	(368)
Cumulative effect of changes in accounting principles				(79)
Net change	(617)	267
Ending balance	(21)	(180)
Accumulated other comprehensive income/(loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(1,507)	(119)
Cumulative effect of changes in accounting principles			$ 0	$ 88
Net change	949	(1,032)
Ending balance	$ (558)	$ (1,063)