Securities - Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
AFS securities
Amortized Cost	$ 29,142	$ 25,349
Gross Unrealized Gains	483	433
Gross Unrealized Losses	176	485
Fair Value	29,449	25,297
HTM securities
Total debt securities	17,750	18,530
Gross Unrealized Gains	357	110
Gross Unrealized Losses	24	121
Fair Value	18,083	18,519
U.S. Treasury
AFS securities
Amortized Cost	2,645	1,836
Gross Unrealized Gains	9	2
Gross Unrealized Losses	4	6
Fair Value	2,650	1,832
HTM securities
Total debt securities	1,098	1,097
Gross Unrealized Gains	49	22
Gross Unrealized Losses	0	0
Fair Value	1,147	1,119
GSE
AFS securities
Amortized Cost	177	51
Gross Unrealized Gains	0	0
Gross Unrealized Losses	1	0
Fair Value	176	51
HTM securities
Total debt securities	2,196	5,045
Gross Unrealized Gains	79	16
Gross Unrealized Losses	0	98
Fair Value	2,275	4,963
Agency MBS
AFS securities
Amortized Cost	23,503	20,463
Gross Unrealized Gains	155	22
Gross Unrealized Losses	123	439
Fair Value	23,535	20,046
HTM securities
Total debt securities	14,282	12,267
Gross Unrealized Gains	228	70
Gross Unrealized Losses	24	22
Fair Value	14,486	12,315
States and political subdivisions
AFS securities
Amortized Cost	2,316	2,312
Gross Unrealized Gains	88	103
Gross Unrealized Losses	48	40
Fair Value	2,356	2,375
HTM securities
Total debt securities	122	63
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	122	63
Non-agency MBS
AFS securities
Amortized Cost	494	683
Gross Unrealized Gains	228	306
Gross Unrealized Losses	0	0
Fair Value	722	989
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	52	58
Gross Unrealized Gains	1	2
Gross Unrealized Losses	0	1
Fair Value	$ 53	$ 59