Securities - Gross Unrealized Losses and Fair Values of Investments in Continuous Unrealized Loss Positions (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
AFS securities, Fair Value
Less than 12 months	$ 6,158	$ 13,327
12 months or more	5,538	5,905
Total	11,696	19,232
AFS securities, Unrealized Losses
Less than 12 months	31	206
12 months or more	166	279
Total	197	485
HTM securities, Fair Value
Less than 12 months	2,154	6,355
12 months or more	865	1,867
Total	3,019	8,222
HTM securities, Unrealized Losses
Less than 12 months	15	63
12 months or more	6	58
Total	21	121
U.S. Treasury
AFS securities, Fair Value
Less than 12 months		1,211
12 months or more		0
Total		1,211
AFS securities, Unrealized Losses
Less than 12 months		6
12 months or more		0
Total		6
Agency MBS
AFS securities, Fair Value
Less than 12 months	6,082	12,052
12 months or more	5,202	5,576
Total	11,284	17,628
AFS securities, Unrealized Losses
Less than 12 months	30	199
12 months or more	117	240
Total	147	439
HTM securities, Fair Value
Less than 12 months	1,553	3,992
12 months or more	170	124
Total	1,723	4,116
HTM securities, Unrealized Losses
Less than 12 months	12	21
12 months or more	1	1
Total	13	22
States and political subdivisions
AFS securities, Fair Value
Less than 12 months	76	64
12 months or more	336	329
Total	412	393
AFS securities, Unrealized Losses
Less than 12 months	1	1
12 months or more	49	39
Total	50	40
GSE
HTM securities, Fair Value
Less than 12 months	548	2,307
12 months or more	695	1,743
Total	1,243	4,050
HTM securities, Unrealized Losses
Less than 12 months	2	41
12 months or more	5	57
Total	7	98
Other
HTM securities, Fair Value
Less than 12 months	53	56
12 months or more	0	0
Total	53	56
HTM securities, Unrealized Losses
Less than 12 months	1	1
12 months or more	0	0
Total	$ 1	$ 1