Accumulated Other Comprehensive Income/(Loss) - Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance			$ 255,693
Net change	$ (75)	$ 531	(1,107)	$ 783
Ending balance	257,458	258,483	257,458	258,483
Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes in accounting principles					$ 261
Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes in accounting principles					115
Reclassification of certain tax effects from AOCI
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes in accounting principles					(288)
Unrealized gains/(losses) on investment securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	1,826	1,762	2,164	1,524
Net change	(227)	457	(1,461)	695
Ending balance	1,599	2,219	1,599	2,219
Unrealized gains/(losses) on investment securities | Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes in accounting principles					261
Unrealized gains/(losses) on investment securities | Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes 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 changes in accounting principles					466
Translation adjustments, net of hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(720)	(157)	(470)	(164)
Net change	88	0	115	7
Ending balance	(632)	(157)	(632)	(157)
Translation adjustments, net of hedges | Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes in accounting principles					0
Translation adjustments, net of hedges | Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes 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 changes in accounting principles					(277)
Fair value hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(94)		0
Net change	(68)		(108)
Ending balance	(162)		(162)
Fair value hedges | Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes in accounting principles					0
Fair value hedges | Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes in accounting principles					(54)
Fair value hedges | Reclassification of certain tax effects from AOCI
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes in accounting principles					0
Cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	19	(9)	76	(100)
Net change	(166)	53	(239)	144
Ending balance	(147)	44	(147)	44
Cash flow hedges | Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes in accounting principles					0
Cash flow hedges | Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes in accounting principles					0
Cash flow hedges | Reclassification of certain tax effects from AOCI
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes in accounting principles					16
Defined benefit pension and OPEB plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(1,914)	(2,274)	(1,521)	(2,259)
Net change	38	19	59	4
Ending balance	(1,876)	(2,255)	(1,876)	(2,255)
Defined benefit pension and OPEB plans | Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes in accounting principles					0
Defined benefit pension and OPEB plans | Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes 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 changes in accounting principles					(414)
DVA on fair value option elected liabilities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(180)	(245)	(368)	(176)
Net change	260	2	527	(67)
Ending balance	80	(243)	80	(243)
DVA on fair value option elected liabilities | Premium amortization on purchased callable debt securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes in accounting principles					0
DVA on fair value option elected liabilities | Hedge accounting
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Cumulative effect of changes 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 changes in accounting principles					(79)
Accumulated other comprehensive income/(loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(1,063)	(923)	(119)	(1,175)
Cumulative effect of changes in accounting principles					$ 88	$ 0
Net change			(1,107)	783
Ending balance	$ (1,138)	$ (392)	$ (1,138)	$ (392)