Securities - Schedule of Fair Values and Gross Unrealized Losses of Investments by Investment Category and Length of Time (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
AFS securities, Fair Value
Less than 12 months	$ 5,921	$ 17,401
12 months or more	16,100	5,452
Total	22,021	22,853
AFS securities, Unrealized Losses
Less than 12 months	69	443
12 months or more	630	268
Total	699	711
HTM securities, Fair Value
Less than 12 months	12,350	9,479
12 months or more	4,921	305
Total	17,271	9,784
HTM securities, Unrealized Losses
Less than 12 months	89	208
12 months or more	155	2
Total	244	210
U.S. Treasury
AFS securities, Fair Value
Less than 12 months	634	2,014
12 months or more	1,655	0
Total	2,289	2,014
AFS securities, Unrealized Losses
Less than 12 months	4	84
12 months or more	73	0
Total	77	84
GSE
AFS securities, Fair Value
Less than 12 months	9	180
12 months or more	170	0
Total	179	180
AFS securities, Unrealized Losses
Less than 12 months	0	10
12 months or more	8	0
Total	8	10
HTM securities, Fair Value
Less than 12 months	1,470	1,762
12 months or more	290	0
Total	1,760	1,762
HTM securities, Unrealized Losses
Less than 12 months	12	30
12 months or more	10	0
Total	22	30
Agency MBS
AFS securities, Fair Value
Less than 12 months	5,077	14,842
12 months or more	13,920	5,138
Total	18,997	19,980
AFS securities, Unrealized Losses
Less than 12 months	64	342
12 months or more	526	226
Total	590	568
HTM securities, Fair Value
Less than 12 months	10,880	7,717
12 months or more	4,631	305
Total	15,511	8,022
HTM securities, Unrealized Losses
Less than 12 months	77	178
12 months or more	145	2
Total	222	180
States and political subdivisions
AFS securities, Fair Value
Less than 12 months	201	365
12 months or more	355	314
Total	556	679
AFS securities, Unrealized Losses
Less than 12 months	1	7
12 months or more	23	42
Total	$ 24	$ 49