Securities - Gross Unrealized Losses and Fair Values of Investments in Continuous Unrealized Loss Positions (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
AFS securities, Fair Value
Less than 12 months	$ 8,201	$ 536
12 months or more	4,521	21,063
Total	12,722	21,599
AFS securities, Unrealized Losses
Less than 12 months	48	3
12 months or more	97	895
Total	145	898
HTM securities, Fair Value
Less than 12 months	1,622	962
12 months or more	1,274	17,120
Total	2,896	18,082
HTM securities, Unrealized Losses
Less than 12 months	9	6
12 months or more	15	530
Total	24	536
U.S. Treasury
AFS securities, Fair Value
Less than 12 months	791	111
12 months or more	0	2,121
Total	791	2,232
AFS securities, Unrealized Losses
Less than 12 months	5	0
12 months or more	0	84
Total	5	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	46	3
12 months or more	78	176
Total	124	179
AFS securities, Unrealized Losses
Less than 12 months	0	0
12 months or more	1	9
Total	1	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	7,285	322
12 months or more	4,299	18,478
Total	11,584	18,800
AFS securities, Unrealized Losses
Less than 12 months	42	2
12 months or more	91	785
Total	133	787
HTM securities, Fair Value
Less than 12 months	1,622	264
12 months or more	1,274	14,976
Total	2,896	15,240
HTM securities, Unrealized Losses
Less than 12 months	9	3
12 months or more	15	484
Total	24	487
States and political subdivisions
AFS securities, Fair Value
Less than 12 months	79	100
12 months or more	144	288
Total	223	388
AFS securities, Unrealized Losses
Less than 12 months	1	1
12 months or more	5	17
Total	$ 6	$ 18