Securities - Gross Unrealized Losses and Fair Values of Investments in Continuous Unrealized Loss Positions (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
AFS securities, Fair Value
Less than 12 months	$ 6,048	$ 13,327
12 months or more	6,049	5,905
Total	12,097	19,232
AFS securities, Unrealized Losses
Less than 12 months	24	206
12 months or more	152	279
Total	176	485
HTM securities, Fair Value
Less than 12 months	1,852	6,355
12 months or more	103	1,867
Total	1,955	8,222
HTM securities, Unrealized Losses
Less than 12 months	23	63
12 months or more	1	58
Total	24	121
U.S. Treasury
AFS securities, Fair Value
Less than 12 months	1,574	1,211
12 months or more	0	0
Total	1,574	1,211
AFS securities, Unrealized Losses
Less than 12 months	4	6
12 months or more	0	0
Total	4	6
GSE
AFS securities, Fair Value
Less than 12 months	122
12 months or more	0
Total	122
AFS securities, Unrealized Losses
Less than 12 months	1
12 months or more	0
Total	1
HTM securities, Fair Value
Less than 12 months		2,307
12 months or more		1,743
Total		4,050
HTM securities, Unrealized Losses
Less than 12 months		41
12 months or more		57
Total		98
Agency MBS
AFS securities, Fair Value
Less than 12 months	4,169	12,052
12 months or more	5,695	5,576
Total	9,864	17,628
AFS securities, Unrealized Losses
Less than 12 months	17	199
12 months or more	106	240
Total	123	439
HTM securities, Fair Value
Less than 12 months	1,852	3,992
12 months or more	103	124
Total	1,955	4,116
HTM securities, Unrealized Losses
Less than 12 months	23	21
12 months or more	1	1
Total	24	22
States and political subdivisions
AFS securities, Fair Value
Less than 12 months	183	64
12 months or more	354	329
Total	537	393
AFS securities, Unrealized Losses
Less than 12 months	2	1
12 months or more	46	39
Total	$ 48	40
Other
HTM securities, Fair Value
Less than 12 months		56
12 months or more		0
Total		56
HTM securities, Unrealized Losses
Less than 12 months		1
12 months or more		0
Total		$ 1