Securities - Gross Unrealized Losses and Fair Values of Investments in Continuous Unrealized Loss Positions (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
AFS securities, Fair Value
Less than 12 months	$ 4,683	$ 5,921
12 months or more	18,312	16,100
Total	22,995	22,021
AFS securities, Unrealized Losses
Less than 12 months	121	69
12 months or more	1,248	630
Total	1,369	699
HTM securities, Fair Value
Less than 12 months	8,853	12,350
12 months or more	9,630	4,921
Total	18,483	17,271
HTM securities, Unrealized Losses
Less than 12 months	305	89
12 months or more	545	155
Total	850	244
U.S. Treasury
AFS securities, Fair Value
Less than 12 months	452	634
12 months or more	1,926	1,655
Total	2,378	2,289
AFS securities, Unrealized Losses
Less than 12 months	8	4
12 months or more	119	73
Total	127	77
HTM securities, Fair Value
Less than 12 months	1,084
12 months or more	0
Total	1,084
HTM securities, Unrealized Losses
Less than 12 months	15
12 months or more	0
Total	15
GSE
AFS securities, Fair Value
Less than 12 months	33	9
12 months or more	165	170
Total	198	179
AFS securities, Unrealized Losses
Less than 12 months	1	0
12 months or more	11	8
Total	12	8
HTM securities, Fair Value
Less than 12 months	900	1,470
12 months or more	1,081	290
Total	1,981	1,760
HTM securities, Unrealized Losses
Less than 12 months	24	12
12 months or more	47	10
Total	71	22
Agency MBS
AFS securities, Fair Value
Less than 12 months	4,052	5,077
12 months or more	15,949	13,920
Total	20,001	18,997
AFS securities, Unrealized Losses
Less than 12 months	111	64
12 months or more	1,102	526
Total	1,213	590
HTM securities, Fair Value
Less than 12 months	6,869	10,880
12 months or more	8,549	4,631
Total	15,418	15,511
HTM securities, Unrealized Losses
Less than 12 months	266	77
12 months or more	498	145
Total	764	222
States and political subdivisions
AFS securities, Fair Value
Less than 12 months	146	201
12 months or more	272	355
Total	418	556
AFS securities, Unrealized Losses
Less than 12 months	1	1
12 months or more	16	23
Total	$ 17	$ 24