Investment Securities - Gross Unrealized Losses and Fair Values of Investments in Continuous Unrealized Loss Positions (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
AFS securities, Fair Value
Less than 12 months	$ 21,646	$ 536
12 months or more	1,594	21,063
Total	23,240	21,599
AFS securities, Unrealized Losses
Less than 12 months	157	3
12 months or more	20	895
Total	177	898
HTM securities, Fair Value
Less than 12 months		962
12 months or more		17,120
Total		18,082
HTM securities, Unrealized Losses
Less than 12 months		6
12 months or more		530
Total		536
U.S. Treasury
AFS securities, Fair Value
Less than 12 months	702	111
12 months or more	0	2,121
Total	702	2,232
AFS securities, Unrealized Losses
Less than 12 months	6	0
12 months or more	0	84
Total	6	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
US Government-sponsored Enterprises Debt Securities [Member]
AFS securities, Fair Value
Less than 12 months	6	3
12 months or more	0	176
Total	6	179
AFS securities, Unrealized Losses
Less than 12 months	0	0
12 months or more	0	9
Total	0	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
States and political subdivisions
AFS securities, Fair Value
Less than 12 months	65	100
12 months or more	144	288
Total	209	388
AFS securities, Unrealized Losses
Less than 12 months	1	1
12 months or more	5	17
Total	6	18
Residential Mortgage Backed Securities
AFS securities, Fair Value
Less than 12 months	20,328	222
12 months or more	1,326	17,892
Total	21,654	18,114
AFS securities, Unrealized Losses
Less than 12 months	145	0
12 months or more	13	758
Total	158	758
HTM securities, Fair Value
Less than 12 months		264
12 months or more		14,976
Total		15,240
HTM securities, Unrealized Losses
Less than 12 months		3
12 months or more		484
Total		487
Commercial Mortgage Backed Securities
AFS securities, Fair Value
Less than 12 months	545	100
12 months or more	124	586
Total	669	686
AFS securities, Unrealized Losses
Less than 12 months	5	2
12 months or more	2	27
Total	$ 7	$ 29