Accumulated Other Comprehensive Income/(Loss) - Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 2,189
Net change, Unrealized gains/(losses) on investment securities	(2,144)	$ 1,975	$ (4,070)
Net change, Translation adjustments, net of hedges	(15)	(11)	(41)
Net change, Cash flow hedges	51	44	(259)
Net change, Defined benefit pension and OPEB plans	111	(1,018)	1,467
Net change, Accumulated other comprehensive income/(loss)	(1,997)	990	(2,903)
Ending balance	192	2,189
Accumulated other comprehensive income (loss) - supplemental information
Net pre-tax unrealized gains (losses) in AOCI on securities on the date of transfer				$ 9
Unrealized gains/(losses) on investment securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	4,773	2,798	6,868
Net change, Unrealized gains/(losses) on investment securities	(2,144)	1,975	(4,070)
Ending balance	2,629	4,773	2,798
Translation adjustments, net of hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(147)	(136)	(95)
Net change, Translation adjustments, net of hedges	(15)	(11)	(41)
Ending balance	(162)	(147)	(136)
Cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(95)	(139)	120
Net change, Cash flow hedges	51	44	(259)
Ending balance	(44)	(95)	(139)
Defined benefit pension and OPEB plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(2,342)	(1,324)	(2,791)
Net change, Defined benefit pension and OPEB plans	111	(1,018)	1,467
Ending balance	(2,231)	(2,342)	(1,324)
Accumulated other comprehensive income/(loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	2,189	1,199	4,102
Net change, Accumulated other comprehensive income/(loss)	(1,997)	990	(2,903)
Ending balance	$ 192	$ 2,189	$ 1,199