Accumulated other comprehensive income/(loss) - Rollforward (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance			$ 2,189
Net change, Unrealized gains/(losses) on investment securities	$ (291)	$ (141)	(1,621)	$ 1,928
Net change, Translation adjustments, net of hedges	(5)	3	(12)	13
Net change, Cash flow hedges	(106)	(58)	51	69
Net change, Defined benefit pension and OPEB plans	51	24	144	57
Net change, Accumulated other comprehensive income/(loss)	(351)	(172)	(1,438)	2,067
Ending Balance	751		751
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	3,443	4,867	4,773	2,798
Net change, Unrealized gains/(losses) on investment securities	(291)	(141)	(1,621)	1,928
Ending Balance	3,152	4,726	3,152	4,726
Translation adjustments, net of hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(154)	(126)	(147)	(136)
Net change, Translation adjustments, net of hedges	(5)	3	(12)	13
Ending Balance	(159)	(123)	(159)	(123)
Cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	62	(12)	(95)	(139)
Net change, Cash flow hedges	(106)	(58)	51	69
Ending Balance	(44)	(70)	(44)	(70)
Defined benefit pension and OPEB plans
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(2,249)	(1,291)	(2,342)	(1,324)
Net change, Defined benefit pension and OPEB plans	51	24	144	57
Ending Balance	(2,198)	(1,267)	(2,198)	(1,267)
Accumulated other comprehensive income
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	1,102	3,438	2,189	1,199
Net change, Accumulated other comprehensive income/(loss)	(351)	(172)	(1,438)	2,067
Ending Balance	$ 751	$ 3,266	$ 751	$ 3,266