Accumulated Other Comprehensive Income/(Loss) - Components of Other Comprehensive Income/(Loss) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Unrealized gains/(losses) on AFS securities:
Net change, After-tax		$ (2,144)	$ 1,975	$ (4,070)
Translation adjustments:
Net change, After-tax		(15)	(11)	(41)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax [Abstract]
Net change, After-tax		51	44	(259)
Defined benefit pension and OPEB plans:
Net change, After-tax		111	(1,018)	1,467
Total other comprehensive income/(loss), after–tax		(1,997)	990	(2,903)
Reclass of net losses from AOCI to other income	$ 150
Unrealized gains/(losses) on investment securities
Unrealized gains/(losses) on AFS securities:
Net unrealized gains/(losses) arising during the period before tax		(3,315)	3,193	(5,987)
Net unrealized gains/(losses) arising during the period tax effect		1,297	(1,170)	2,323
Net unrealized gains/(losses) arising during the period after tax		(2,018)	2,023	(3,664)
Reclassification adjustment for realized (gains)/losses included in net income before tax		(202)	(77)	(667)
Reclassification adjustment for realized (gains)/losses included in net income tax effect		76	29	261
Reclassification adjustment for realized (gains)/losses included in net income after tax		(126)	(48)	(406)
Net change before tax		(3,517)	3,116	(6,654)
Net change tax effect		1,373	(1,141)	2,584
Net change, After-tax		(2,144)	1,975	(4,070)
Translation adjustments, net of hedges
Translation adjustments:
Translation before tax		(1,876)	(1,638)	(807)
Translation tax effect		682	588	295
Translation after tax		(1,194)	(1,050)	(512)
Hedges before tax		1,885	1,698	773
Hedges tax effect		(706)	(659)	(302)
Hedges after tax		1,179	1,039	471
Net change before tax		9	60	(34)
Net change tax effect		(24)	(71)	(7)
Net change, After-tax		(15)	(11)	(41)
Cash flow hedges
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax [Abstract]
Net unrealized gains/(losses) arising during the period before tax		(97)	98	(525)
Net unrealized gains/(losses) arising during the period tax effect		35	(39)	206
Net unrealized gains/(losses) arising during the period after tax		(62)	59	(319)
Reclassification adjustment for realized (gains)/losses included in net income before tax		180	(24)	101
Reclassification adjustment for realized (gains)/losses included in net income tax effect		(67)	9	(41)
Reclassification adjustment for realized (gains)/losses included in net income after tax		113	(15)	60
Net change before tax		83	74	(424)
Net change tax effect		(32)	(30)	165
Net change, After-tax		51	44	(259)
Defined benefit pension and OPEB plans
Defined benefit pension and OPEB plans:
Prior service credits arising during the period before tax		0	(53)	0
Prior service credits arising during the period tax effect		0	21	0
Prior service credits arising during the period after tax		0	(32)	0
Net gains/(losses) arising during the period before tax		29	(1,697)	2,055
Net gains/(losses) arising during the period tax effect		(47)	688	(750)
Net gains/(losses) arising during the period after tax		(18)	(1,009)	1,305
Reclassification adjustments included in net income, amortization of net loss before tax		282	72	321
Reclassification adjustments included in net income, amortization of net loss tax effect		(106)	(29)	(124)
Reclassification adjustments included in net income, amortization of net loss after tax		176	43	197
Reclassification adjustments included in net income, prior service costs/(credits) before tax		(36)	(44)	(43)
Reclassification adjustments included in net income, prior service costs/(credits) tax effect		14	17	17
Reclassification adjustments included in net income, prior service costs/(credits) after tax		(22)	(27)	(26)
Foreign exchange and other before tax		33	39	(14)
Foreign exchange and other tax effect		(58)	(32)	5
Foreign exchange and other after tax		(25)	7	(9)
Net change before tax		308	(1,683)	2,319
Net change tax effect		(197)	665	(852)
Net change, After-tax		111	(1,018)	1,467
Accumulated other comprehensive income/(loss)
Defined benefit pension and OPEB plans:
Total other comprehensive income/(loss) before tax		(3,117)	1,567	(4,793)
Total other comprehensive income/(loss) tax effect		1,120	(577)	1,890
Total other comprehensive income/(loss), after–tax		$ (1,997)	$ 990	$ (2,903)