Securities - Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
AFS securities
Amortized Cost	$ 25,831	$ 25,009
Gross Unrealized Gains	247	237
Gross Unrealized Losses	1,061	699
Fair Value	25,017	24,547
HTM securities
Total debt securities	22,390	23,027
Gross Unrealized Gains	35	54
Gross Unrealized Losses	596	244
Fair Value	21,829	22,837
U.S. Treasury
AFS securities
Amortized Cost	2,441	2,368
Gross Unrealized Gains	0	0
Gross Unrealized Losses	106	77
Fair Value	2,335	2,291
HTM securities
Total debt securities	1,098	1,098
Gross Unrealized Gains	0	8
Gross Unrealized Losses	4	0
Fair Value	1,094	1,106
GSE
AFS securities
Amortized Cost	187	187
Gross Unrealized Gains	0	0
Gross Unrealized Losses	11	8
Fair Value	176	179
HTM securities
Total debt securities	2,198	2,198
Gross Unrealized Gains	5	11
Gross Unrealized Losses	50	22
Fair Value	2,153	2,187
Agency MBS
AFS securities
Amortized Cost	21,605	20,683
Gross Unrealized Gains	3	8
Gross Unrealized Losses	921	590
Fair Value	20,687	20,101
HTM securities
Total debt securities	19,069	19,660
Gross Unrealized Gains	30	33
Gross Unrealized Losses	542	222
Fair Value	18,557	19,471
States and political subdivisions
AFS securities
Amortized Cost	1,194	1,379
Gross Unrealized Gains	28	37
Gross Unrealized Losses	23	24
Fair Value	1,199	1,392
HTM securities
Total debt securities	24	28
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	24	28
Non-agency MBS
AFS securities
Amortized Cost	363	384
Gross Unrealized Gains	215	192
Gross Unrealized Losses	0	0
Fair Value	578	576
Other
AFS securities
Amortized Cost	41	8
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Fair Value	42	8
HTM securities
Total debt securities	1	43
Gross Unrealized Gains	0	2
Gross Unrealized Losses	0	0
Fair Value	$ 1	$ 45