Securities - Gross Unrealized Losses and Fair Values of Investments in Continuous Unrealized Loss Positions (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
AFS securities, Fair Value
Less than 12 months	$ 8,213	$ 5,921
12 months or more	15,420	16,100
Total	23,633	22,021
AFS securities, Unrealized Losses
Less than 12 months	208	69
12 months or more	853	630
Total	1,061	699
HTM securities, Fair Value
Less than 12 months	14,945	12,350
12 months or more	4,626	4,921
Total	19,571	17,271
HTM securities, Unrealized Losses
Less than 12 months	354	89
12 months or more	242	155
Total	596	244
U.S. Treasury
AFS securities, Fair Value
Less than 12 months	697	634
12 months or more	1,637	1,655
Total	2,334	2,289
AFS securities, Unrealized Losses
Less than 12 months	9	4
12 months or more	97	73
Total	106	77
HTM securities, Fair Value
Less than 12 months	1,094
12 months or more	0
Total	1,094
HTM securities, Unrealized Losses
Less than 12 months	4
12 months or more	0
Total	4
GSE
AFS securities, Fair Value
Less than 12 months	9	9
12 months or more	167	170
Total	176	179
AFS securities, Unrealized Losses
Less than 12 months	0	0
12 months or more	11	8
Total	11	8
HTM securities, Fair Value
Less than 12 months	1,455	1,470
12 months or more	287	290
Total	1,742	1,760
HTM securities, Unrealized Losses
Less than 12 months	37	12
12 months or more	13	10
Total	50	22
Agency MBS
AFS securities, Fair Value
Less than 12 months	7,304	5,077
12 months or more	13,273	13,920
Total	20,577	18,997
AFS securities, Unrealized Losses
Less than 12 months	198	64
12 months or more	723	526
Total	921	590
HTM securities, Fair Value
Less than 12 months	12,396	10,880
12 months or more	4,339	4,631
Total	16,735	15,511
HTM securities, Unrealized Losses
Less than 12 months	313	77
12 months or more	229	145
Total	542	222
States and political subdivisions
AFS securities, Fair Value
Less than 12 months	203	201
12 months or more	343	355
Total	546	556
AFS securities, Unrealized Losses
Less than 12 months	1	1
12 months or more	22	23
Total	$ 23	$ 24