Accumulated Other Comprehensive Income/(Loss) - Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 247,573	$ 231,727
Year ended December 31, (in millions)	(1,521)	(1,997)	$ 990
Ending balance	254,190	247,573	231,727
Unrealized gains/(losses) on investment securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	2,629	4,773	2,798
Year ended December 31, (in millions)	(1,105)	(2,144)	1,975
Cumulative effect of change in accounting principle		0
Ending balance	1,524	2,629	4,773
Translation adjustments, net of hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(162)	(147)	(136)
Year ended December 31, (in millions)	(2)	(15)	(11)
Cumulative effect of change in accounting principle		0
Ending balance	(164)	(162)	(147)
Cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(44)	(95)	(139)
Year ended December 31, (in millions)	(56)	51	44
Cumulative effect of change in accounting principle		0
Ending balance	(100)	(44)	(95)
Defined benefit pension and OPEB plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(2,231)	(2,342)	(1,324)
Year ended December 31, (in millions)	(28)	111	(1,018)
Cumulative effect of change in accounting principle		0
Ending balance	(2,259)	(2,231)	(2,342)
DVA on fair value option elected liabilities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Year ended December 31, (in millions)	(330)	0	0
Cumulative effect of change in accounting principle		154
Ending balance	(176)
Accumulated other comprehensive income/(loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	192	2,189	1,199
Year ended December 31, (in millions)	(1,521)	(1,997)	990
Cumulative effect of change in accounting principle		154	0	$ 0
Ending balance	$ (1,175)	$ 192	$ 2,189