Securities - Gross Unrealized Losses and Fair Values of Investments in Continuous Unrealized Loss Positions (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
AFS securities, Fair Value
Less than 12 months	$ 645	$ 536
12 months or more	11,253	21,063
Total	11,898	21,599
AFS securities, Unrealized Losses
Less than 12 months	2	3
12 months or more	192	895
Total	194	898
HTM securities, Fair Value
Less than 12 months	384	962
12 months or more	4,461	17,120
Total	4,845	18,082
HTM securities, Unrealized Losses
Less than 12 months	3	6
12 months or more	53	530
Total	56	536
U.S. Treasury
AFS securities, Fair Value
Less than 12 months	0	111
12 months or more	666	2,121
Total	666	2,232
AFS securities, Unrealized Losses
Less than 12 months	0	0
12 months or more	9	84
Total	9	84
HTM securities, Fair Value
Less than 12 months		698
12 months or more		395
Total		1,093
HTM securities, Unrealized Losses
Less than 12 months		3
12 months or more		3
Total		6
GSE
AFS securities, Fair Value
Less than 12 months	0	3
12 months or more	171	176
Total	171	179
AFS securities, Unrealized Losses
Less than 12 months	0	0
12 months or more	2	9
Total	2	9
HTM securities, Fair Value
Less than 12 months		0
12 months or more		1,749
Total		1,749
HTM securities, Unrealized Losses
Less than 12 months		0
12 months or more		43
Total		43
Agency MBS
AFS securities, Fair Value
Less than 12 months	544	322
12 months or more	10,219	18,478
Total	10,763	18,800
AFS securities, Unrealized Losses
Less than 12 months	1	2
12 months or more	170	785
Total	171	787
HTM securities, Fair Value
Less than 12 months	384	264
12 months or more	4,461	14,976
Total	4,845	15,240
HTM securities, Unrealized Losses
Less than 12 months	3	3
12 months or more	53	484
Total	56	487
States and political subdivisions
AFS securities, Fair Value
Less than 12 months	101	100
12 months or more	197	288
Total	298	388
AFS securities, Unrealized Losses
Less than 12 months	1	1
12 months or more	11	17
Total	$ 12	$ 18