Securities - Gross Unrealized Losses and Fair Values of Investments in Continuous Unrealized Loss Positions (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
AFS securities, Fair Value
Less than 12 months	$ 7,973	$ 5,921
12 months or more	14,747	16,100
Total	22,720	22,021
AFS securities, Unrealized Losses
Less than 12 months	256	69
12 months or more	921	630
Total	1,177	699
HTM securities, Fair Value
Less than 12 months	14,575	12,350
12 months or more	4,537	4,921
Total	19,112	17,271
HTM securities, Unrealized Losses
Less than 12 months	436	89
12 months or more	265	155
Total	701	244
U.S. Treasury
AFS securities, Fair Value
Less than 12 months	655	634
12 months or more	1,643	1,655
Total	2,298	2,289
AFS securities, Unrealized Losses
Less than 12 months	10	4
12 months or more	104	73
Total	114	77
HTM securities, Fair Value
Less than 12 months	1,089
12 months or more	0
Total	1,089
HTM securities, Unrealized Losses
Less than 12 months	9
12 months or more	0
Total	9
GSE
AFS securities, Fair Value
Less than 12 months	9	9
12 months or more	166	170
Total	175	179
AFS securities, Unrealized Losses
Less than 12 months	0	0
12 months or more	11	8
Total	11	8
HTM securities, Fair Value
Less than 12 months	1,446	1,470
12 months or more	286	290
Total	1,732	1,760
HTM securities, Unrealized Losses
Less than 12 months	46	12
12 months or more	14	10
Total	60	22
Agency MBS
AFS securities, Fair Value
Less than 12 months	7,148	5,077
12 months or more	12,624	13,920
Total	19,772	18,997
AFS securities, Unrealized Losses
Less than 12 months	245	64
12 months or more	789	526
Total	1,034	590
HTM securities, Fair Value
Less than 12 months	12,040	10,880
12 months or more	4,251	4,631
Total	16,291	15,511
HTM securities, Unrealized Losses
Less than 12 months	381	77
12 months or more	251	145
Total	632	222
States and political subdivisions
AFS securities, Fair Value
Less than 12 months	161	201
12 months or more	314	355
Total	475	556
AFS securities, Unrealized Losses
Less than 12 months	1	1
12 months or more	17	23
Total	$ 18	$ 24