Securities - Gross Unrealized Losses and Fair Values of Investments in Continuous Unrealized Loss Positions (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
AFS securities, Fair Value
Less than 12 months	$ 1,270	$ 536
12 months or more	18,541	21,063
Total	19,811	21,599
AFS securities, Unrealized Losses
Less than 12 months	2	3
12 months or more	527	895
Total	529	898
HTM securities, Fair Value
Less than 12 months	1,521	962
12 months or more	16,012	17,120
Total	17,533	18,082
HTM securities, Unrealized Losses
Less than 12 months	5	6
12 months or more	235	530
Total	240	536
U.S. Treasury
AFS securities, Fair Value
Less than 12 months	49	111
12 months or more	1,442	2,121
Total	1,491	2,232
AFS securities, Unrealized Losses
Less than 12 months	0	0
12 months or more	43	84
Total	43	84
HTM securities, Fair Value
Less than 12 months	400	698
12 months or more	496	395
Total	896	1,093
HTM securities, Unrealized Losses
Less than 12 months	0	3
12 months or more	2	3
Total	2	6
GSE
AFS securities, Fair Value
Less than 12 months	12	3
12 months or more	179	176
Total	191	179
AFS securities, Unrealized Losses
Less than 12 months	0	0
12 months or more	6	9
Total	6	9
HTM securities, Fair Value
Less than 12 months	0	0
12 months or more	1,768	1,749
Total	1,768	1,749
HTM securities, Unrealized Losses
Less than 12 months	0	0
12 months or more	15	43
Total	15	43
Agency MBS
AFS securities, Fair Value
Less than 12 months	1,165	322
12 months or more	16,679	18,478
Total	17,844	18,800
AFS securities, Unrealized Losses
Less than 12 months	2	2
12 months or more	466	785
Total	468	787
HTM securities, Fair Value
Less than 12 months	1,121	264
12 months or more	13,748	14,976
Total	14,869	15,240
HTM securities, Unrealized Losses
Less than 12 months	5	3
12 months or more	218	484
Total	223	487
States and political subdivisions
AFS securities, Fair Value
Less than 12 months	44	100
12 months or more	241	288
Total	285	388
AFS securities, Unrealized Losses
Less than 12 months	0	1
12 months or more	12	17
Total	$ 12	$ 18