AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	$ (1,028)	$ (751)
OCI before reclassifications, net of tax	123	(1)
Amounts reclassified from AOCI:
Personnel expense	(915)	(830)
Interest income	1,721	1,493
Interest expense	(192)	(181)
FDIC loss share income, net	(60)	(79)
(Gain) loss on securities, net	45	0
Income before income taxes	816	788
Less: Income taxes	246	241
Net income	570	547
Net change in AOCI	111	18
AOCI, ending balance	(917)	(733)
Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Personnel expense	17	12
Interest income	(26)	(15)
Interest expense	17	21
FDIC loss share income, net	16	13
(Gain) loss on securities, net	(45)
Income before income taxes	(21)	31
Less: Income taxes	(9)	12
Net income	(12)	19
Unrecognized Net Pension and Postretirement Costs
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(723)	(626)
OCI before reclassifications, net of tax	0	2
Amounts reclassified from AOCI:
Net change in AOCI	11	9
AOCI, ending balance	(712)	(617)
Unrecognized Net Pension and Postretirement Costs | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Personnel expense	17	12
Interest income	0	0
Interest expense	0	0
FDIC loss share income, net	0	0
(Gain) loss on securities, net	0
Income before income taxes	17	12
Less: Income taxes	6	5
Net income	11	7
Unrealized Net Gains (Losses) on Cash Flow Hedges
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(83)	(54)
OCI before reclassifications, net of tax	(126)	(67)
Amounts reclassified from AOCI:
Net change in AOCI	(115)	(54)
AOCI, ending balance	(198)	(108)
Unrealized Net Gains (Losses) on Cash Flow Hedges | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Personnel expense	0	0
Interest income	0	0
Interest expense	17	21
FDIC loss share income, net	0	0
(Gain) loss on securities, net	0
Income before income taxes	17	21
Less: Income taxes	6	8
Net income	11	13
Unrealized Net Gains (Losses) on AFS Securities
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(34)	152
OCI before reclassifications, net of tax	242	67
Amounts reclassified from AOCI:
Net change in AOCI	197	57
AOCI, ending balance	163	209
Unrealized Net Gains (Losses) on AFS Securities | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Personnel expense	0	0
Interest income	(27)	(16)
Interest expense	0	0
FDIC loss share income, net	0	0
(Gain) loss on securities, net	(45)
Income before income taxes	(72)	(16)
Less: Income taxes	(27)	(6)
Net income	(45)	(10)
FDIC's Share of Unrealized (Gains) Losses on AFS Securities
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(169)	(207)
OCI before reclassifications, net of tax	5	2
Amounts reclassified from AOCI:
Net change in AOCI	15	10
AOCI, ending balance	(154)	(197)
FDIC's Share of Unrealized (Gains) Losses on AFS Securities | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Personnel expense	0	0
Interest income	0	0
Interest expense	0	0
FDIC loss share income, net	16	13
(Gain) loss on securities, net	0
Income before income taxes	16	13
Less: Income taxes	6	5
Net income	10	8
Other, net
AOCI, Net of Tax [Roll Forward]
AOCI, beginning balance	(19)	(16)
OCI before reclassifications, net of tax	2	(5)
Amounts reclassified from AOCI:
Net change in AOCI	3	(4)
AOCI, ending balance	(16)	(20)
Other, net | Amounts reclassified from AOCI
Amounts reclassified from AOCI:
Personnel expense	0	0
Interest income	1	1
Interest expense	0	0
FDIC loss share income, net	0	0
(Gain) loss on securities, net	0
Income before income taxes	1	1
Less: Income taxes	0	0
Net income	$ 1	$ 1