Securities - Gross Unrealized Losses and Fair Values of Investments in Continuous Unrealized Loss Positions (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
AFS securities, Fair Value
Less than 12 months	$ 9,091	$ 17,401
12 months or more	11,612	5,452
Total	20,703	22,853
AFS securities, Unrealized Losses
Less than 12 months	120	443
12 months or more	405	268
Total	525	711
HTM securities, Fair Value
Less than 12 months	10,363	9,479
12 months or more	1,882	305
Total	12,245	9,784
HTM securities, Unrealized Losses
Less than 12 months	85	208
12 months or more	68	2
Total	153	210
U.S. Treasury
AFS securities, Fair Value
Less than 12 months	459	2,014
12 months or more	1,553	0
Total	2,012	2,014
AFS securities, Unrealized Losses
Less than 12 months	2	84
12 months or more	57	0
Total	59	84
GSE
AFS securities, Fair Value
Less than 12 months	12	180
12 months or more	170	0
Total	182	180
AFS securities, Unrealized Losses
Less than 12 months	0	10
12 months or more	6	0
Total	6	10
HTM securities, Fair Value
Less than 12 months	1,185	1,762
12 months or more	146	0
Total	1,331	1,762
HTM securities, Unrealized Losses
Less than 12 months	9	30
12 months or more	4	0
Total	13	30
Agency MBS
AFS securities, Fair Value
Less than 12 months	8,533	14,842
12 months or more	9,481	5,138
Total	18,014	19,980
AFS securities, Unrealized Losses
Less than 12 months	118	342
12 months or more	309	226
Total	427	568
HTM securities, Fair Value
Less than 12 months	9,178	7,717
12 months or more	1,736	305
Total	10,914	8,022
HTM securities, Unrealized Losses
Less than 12 months	76	178
12 months or more	64	2
Total	140	180
States and political subdivisions
AFS securities, Fair Value
Less than 12 months	87	365
12 months or more	408	314
Total	495	679
AFS securities, Unrealized Losses
Less than 12 months	0	7
12 months or more	33	42
Total	$ 33	$ 49