Securities - Summary of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Approximate Fair Values of Securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
AFS securities:
Amortized Cost	$ 25,009	$ 27,333
Gross Unrealized Gains	237	304
Gross Unrealized Losses	699	711
Fair Value	24,547	26,926
HTM securities:
Amortized Cost	23,027	16,680
Gross Unrealized Gains	54	76
Gross Unrealized Losses	244	210
Fair Value	22,837	16,546
U.S. Treasury
AFS securities:
Amortized Cost	2,368	2,669
Gross Unrealized Gains	0	2
Gross Unrealized Losses	77	84
Fair Value	2,291	2,587
HTM securities:
Amortized Cost	1,098	1,098
Gross Unrealized Gains	8	20
Gross Unrealized Losses	0	0
Fair Value	1,106	1,118
GSE
AFS securities:
Amortized Cost	187	190
Gross Unrealized Gains	0	0
Gross Unrealized Losses	8	10
Fair Value	179	180
HTM securities:
Amortized Cost	2,198	2,197
Gross Unrealized Gains	11	14
Gross Unrealized Losses	22	30
Fair Value	2,187	2,181
Agency MBS
AFS securities:
Amortized Cost	20,683	21,819
Gross Unrealized Gains	8	13
Gross Unrealized Losses	590	568
Fair Value	20,101	21,264
HTM securities:
Amortized Cost	19,660	13,225
Gross Unrealized Gains	33	40
Gross Unrealized Losses	222	180
Fair Value	19,471	13,085
States and political subdivisions
AFS securities:
Amortized Cost	1,379	2,198
Gross Unrealized Gains	37	56
Gross Unrealized Losses	24	49
Fair Value	1,392	2,205
HTM securities:
Amortized Cost	28	110
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	28	110
Non-agency MBS
AFS securities:
Amortized Cost	384	446
Gross Unrealized Gains	192	233
Gross Unrealized Losses	0	0
Fair Value	576	679
Other
AFS securities:
Amortized Cost	8	11
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	8	11
HTM securities:
Amortized Cost	43	50
Gross Unrealized Gains	2	2
Gross Unrealized Losses	0	0
Fair Value	$ 45	$ 52