Securities - Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
AFS securities
Amortized Cost	$ 27,820	$ 25,349
Gross Unrealized Gains	554	433
Gross Unrealized Losses	130	485
Fair Value	28,244	25,297
HTM securities
Total debt securities	18,762	18,530
Gross Unrealized Gains	405	110
Gross Unrealized Losses	10	121
Fair Value	19,157	18,519
U.S. Treasury
AFS securities
Amortized Cost	1,159	1,836
Gross Unrealized Gains	14	2
Gross Unrealized Losses	0	6
Fair Value	1,173	1,832
HTM securities
Total debt securities	1,097	1,097
Gross Unrealized Gains	59	22
Gross Unrealized Losses	0	0
Fair Value	1,156	1,119
Agency MBS
AFS securities
Amortized Cost	23,727	20,463
Gross Unrealized Gains	199	22
Gross Unrealized Losses	87	439
Fair Value	23,839	20,046
HTM securities
Total debt securities	13,531	12,267
Gross Unrealized Gains	255	70
Gross Unrealized Losses	9	22
Fair Value	13,777	12,315
States and political subdivisions
AFS securities
Amortized Cost	2,153	2,025
Gross Unrealized Gains	101	94
Gross Unrealized Losses	43	40
Fair Value	2,211	2,079
HTM securities
Total debt securities	134	63
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	134	63
Non-agency MBS
AFS securities
Amortized Cost	68	198
Gross Unrealized Gains	0	23
Gross Unrealized Losses	0	0
Fair Value	68	221
Other
AFS securities
Amortized Cost	7	4
Gross Unrealized Gains	3	0
Gross Unrealized Losses	0	0
Fair Value	10	4
HTM securities
Total debt securities	54	58
Gross Unrealized Gains	2	2
Gross Unrealized Losses	1	1
Fair Value	55	59
Acquired from FDIC
AFS securities
Amortized Cost	706	772
Gross Unrealized Gains	237	292
Gross Unrealized Losses	0	0
Fair Value	943	1,064
GSE
AFS securities
Amortized Cost		51
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		51
HTM securities
Total debt securities	3,946	5,045
Gross Unrealized Gains	89	16
Gross Unrealized Losses	0	98
Fair Value	$ 4,035	$ 4,963