Securities - Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
AFS securities
Amortized Cost	$ 26,866	$ 25,349
Gross Unrealized Gains	460	433
Gross Unrealized Losses	197	485
Fair Value	27,129	25,297
HTM securities
Total debt securities	19,351	18,530
Gross Unrealized Gains	291	110
Gross Unrealized Losses	21	121
Fair Value	19,621	18,519
U.S. Treasury
AFS securities
Amortized Cost	1,061	1,836
Gross Unrealized Gains	8	2
Gross Unrealized Losses	0	6
Fair Value	1,069	1,832
HTM securities
Total debt securities	1,097	1,097
Gross Unrealized Gains	50	22
Gross Unrealized Losses	0	0
Fair Value	1,147	1,119
Agency MBS
AFS securities
Amortized Cost	22,974	20,463
Gross Unrealized Gains	89	22
Gross Unrealized Losses	147	439
Fair Value	22,916	20,046
HTM securities
Total debt securities	13,443	12,267
Gross Unrealized Gains	182	70
Gross Unrealized Losses	13	22
Fair Value	13,612	12,315
States and political subdivisions
AFS securities
Amortized Cost	2,009	2,025
Gross Unrealized Gains	97	94
Gross Unrealized Losses	50	40
Fair Value	2,056	2,079
HTM securities
Total debt securities	60	63
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	60	63
Non-agency MBS
AFS securities
Amortized Cost	73	198
Gross Unrealized Gains	0	23
Gross Unrealized Losses	0	0
Fair Value	73	221
Other
AFS securities
Amortized Cost	5	4
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	5	4
HTM securities
Total debt securities	56	58
Gross Unrealized Gains	2	2
Gross Unrealized Losses	1	1
Fair Value	57	59
Acquired from FDIC
AFS securities
Amortized Cost	744	772
Gross Unrealized Gains	266	292
Gross Unrealized Losses	0	0
Fair Value	1,010	1,064
GSE
AFS securities
Amortized Cost		51
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		51
HTM securities
Total debt securities	4,695	5,045
Gross Unrealized Gains	57	16
Gross Unrealized Losses	7	98
Fair Value	$ 4,745	$ 4,963