Accumulated Other Comprehensive Income/(Loss) - Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance			$ 255,693
Net change	$ (1,287)	$ 83	(2,394)	$ 866
Ending balance	258,956	258,382	258,956	258,382
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	1,599	2,219	2,164	1,524
Net change	(819)	147	(2,280)	842
Ending balance	780	2,366	780	2,366
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	(632)	(157)	(470)	(164)
Net change	(31)	0	84	7
Ending balance	(663)	(157)	(663)	(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	(162)		0
Net change	34		(74)
Ending balance	(128)		(128)
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	(147)	44	76	(100)
Net change	(88)	26	(327)	170
Ending balance	(235)	70	(235)	70
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	(1,876)	(2,255)	(1,521)	(2,259)
Net change	19	22	78	26
Ending balance	(1,857)	(2,233)	(1,857)	(2,233)
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	80	(243)	(368)	(176)
Net change	(402)	(112)	125	(179)
Ending balance	(322)	(355)	(322)	(355)
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	(1,138)	(392)	(119)	(1,175)
Cumulative effect of changes in accounting principles					$ 88	$ 0
Net change			(2,394)	866
Ending balance	$ (2,425)	$ (309)	$ (2,425)	$ (309)