Investment Securities - Gross Unrealized Losses and Fair Values of Investments in Continuous Unrealized Loss Positions (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
AFS securities, Fair Value
Less than 12 months	$ 1,390	$ 21,646
12 months or more	418	1,594
Total	1,808	23,240
AFS securities, Unrealized Losses
Less than 12 months	29	157
12 months or more	17	20
Total	46	177
U.S. Treasury
AFS securities, Fair Value
Less than 12 months		702
12 months or more		0
Total		702
AFS securities, Unrealized Losses
Less than 12 months		6
12 months or more		0
Total		6
US Government-sponsored Enterprises Debt Securities [Member]
AFS securities, Fair Value
Less than 12 months	3	6
12 months or more	0	0
Total	3	6
AFS securities, Unrealized Losses
Less than 12 months	0	0
12 months or more	0	0
Total	0	0
Agency MBS | Residential Mortgage Backed Securities
AFS securities, Fair Value
Less than 12 months	1,147	20,328
12 months or more	262	1,326
Total	1,409	21,654
AFS securities, Unrealized Losses
Less than 12 months	13	145
12 months or more	9	13
Total	22	158
Agency MBS | Commercial Mortgage Backed Securities
AFS securities, Fair Value
Less than 12 months	66	545
12 months or more	35	124
Total	101	669
AFS securities, Unrealized Losses
Less than 12 months	0	5
12 months or more	1	2
Total	1	7
States and political subdivisions
AFS securities, Fair Value
Less than 12 months	96	65
12 months or more	121	144
Total	217	209
AFS securities, Unrealized Losses
Less than 12 months	1	1
12 months or more	7	5
Total	8	$ 6
Non-agency MBS
AFS securities, Fair Value
Less than 12 months	42
12 months or more	0
Total	42
AFS securities, Unrealized Losses
Less than 12 months	14
12 months or more	0
Total	14
Other
AFS securities, Fair Value
Less than 12 months	36
12 months or more	0
Total	36
AFS securities, Unrealized Losses
Less than 12 months	1
12 months or more	0
Total	$ 1