Securities - Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
AFS securities
Amortized Cost	$ 23,450	$ 27,333
Gross Unrealized Gains	259	304
Gross Unrealized Losses	525	711
Fair Value	23,184	26,926
HTM securities
Total debt securities	23,447	16,680
Gross Unrealized Gains	98	76
Gross Unrealized Losses	153	210
Fair Value	23,392	16,546
U.S. Treasury
AFS securities
Amortized Cost	2,170	2,669
Gross Unrealized Gains	0	2
Gross Unrealized Losses	59	84
Fair Value	2,111	2,587
HTM securities
Total debt securities	1,098	1,098
Gross Unrealized Gains	18	20
Gross Unrealized Losses	0	0
Fair Value	1,116	1,118
GSE
AFS securities
Amortized Cost	188	190
Gross Unrealized Gains	0	0
Gross Unrealized Losses	6	10
Fair Value	182	180
HTM securities
Total debt securities	2,197	2,197
Gross Unrealized Gains	16	14
Gross Unrealized Losses	13	30
Fair Value	2,200	2,181
Agency MBS
AFS securities
Amortized Cost	19,096	21,819
Gross Unrealized Gains	7	13
Gross Unrealized Losses	427	568
Fair Value	18,676	21,264
HTM securities
Total debt securities	20,073	13,225
Gross Unrealized Gains	62	40
Gross Unrealized Losses	140	180
Fair Value	19,995	13,085
States and political subdivisions
AFS securities
Amortized Cost	1,586	2,198
Gross Unrealized Gains	46	56
Gross Unrealized Losses	33	49
Fair Value	1,599	2,205
HTM securities
Total debt securities	34	110
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	34	110
Non-agency MBS
AFS securities
Amortized Cost	402	446
Gross Unrealized Gains	206	233
Gross Unrealized Losses	0	0
Fair Value	608	679
Other
AFS securities
Amortized Cost	8	11
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	8	11
HTM securities
Total debt securities	45	50
Gross Unrealized Gains	2	2
Gross Unrealized Losses	0	0
Fair Value	$ 47	$ 52