Securities - Gross Unrealized Losses and Fair Values of Investments in Continuous Unrealized Loss Positions (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
AFS securities, Fair Value
Less than 12 months	$ 536	$ 5,921
12 months or more	21,063	16,100
Total	21,599	22,021
AFS securities, Unrealized Losses
Less than 12 months	3	69
12 months or more	895	630
Total	898	699
HTM securities, Fair Value
Less than 12 months	962	12,350
12 months or more	17,120	4,921
Total	18,082	17,271
HTM securities, Unrealized Losses
Less than 12 months	6	89
12 months or more	530	155
Total	536	244
U.S. Treasury
AFS securities, Fair Value
Less than 12 months	111	634
12 months or more	2,121	1,655
Total	2,232	2,289
AFS securities, Unrealized Losses
Less than 12 months	0	4
12 months or more	84	73
Total	84	77
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	3	9
12 months or more	176	170
Total	179	179
AFS securities, Unrealized Losses
Less than 12 months	0	0
12 months or more	9	8
Total	9	8
HTM securities, Fair Value
Less than 12 months	0	1,470
12 months or more	1,749	290
Total	1,749	1,760
HTM securities, Unrealized Losses
Less than 12 months	0	12
12 months or more	43	10
Total	43	22
Agency MBS
AFS securities, Fair Value
Less than 12 months	322	5,077
12 months or more	18,478	13,920
Total	18,800	18,997
AFS securities, Unrealized Losses
Less than 12 months	2	64
12 months or more	785	526
Total	787	590
HTM securities, Fair Value
Less than 12 months	264	10,880
12 months or more	14,976	4,631
Total	15,240	15,511
HTM securities, Unrealized Losses
Less than 12 months	3	77
12 months or more	484	145
Total	487	222
States and political subdivisions
AFS securities, Fair Value
Less than 12 months	100	201
12 months or more	288	355
Total	388	556
AFS securities, Unrealized Losses
Less than 12 months	1	1
12 months or more	17	23
Total	$ 18	$ 24