Securities - Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
AFS securities
AFS securities, Amortized Cost	$ 24,863	$ 25,009
Gross Unrealized Gains	233	237
Gross Unrealized Losses	1,177	699
AFS securities, Fair Value	23,919	24,547
HTM securities
HTM securities, Amortized Cost	21,749	23,027
Gross Unrealized Gains	32	54
Gross Unrealized Losses	701	244
HTM securities, Fair Value	21,080	22,837
U.S. Treasury
AFS securities
AFS securities, Amortized Cost	2,437	2,368
Gross Unrealized Gains	0	0
Gross Unrealized Losses	114	77
AFS securities, Fair Value	2,323	2,291
HTM securities
HTM securities, Amortized Cost	1,098	1,098
Gross Unrealized Gains	0	8
Gross Unrealized Losses	9	0
HTM securities, Fair Value	1,089	1,106
GSE
AFS securities
AFS securities, Amortized Cost	186	187
Gross Unrealized Gains	0	0
Gross Unrealized Losses	11	8
AFS securities, Fair Value	175	179
HTM securities
HTM securities, Amortized Cost	2,198	2,198
Gross Unrealized Gains	2	11
Gross Unrealized Losses	60	22
HTM securities, Fair Value	2,140	2,187
Agency MBS
AFS securities
AFS securities, Amortized Cost	20,880	20,683
Gross Unrealized Gains	2	8
Gross Unrealized Losses	1,034	590
AFS securities, Fair Value	19,848	20,101
HTM securities
HTM securities, Amortized Cost	18,436	19,660
Gross Unrealized Gains	30	33
Gross Unrealized Losses	632	222
HTM securities, Fair Value	17,834	19,471
States and political subdivisions
AFS securities
AFS securities, Amortized Cost	971	1,379
Gross Unrealized Gains	27	37
Gross Unrealized Losses	18	24
AFS securities, Fair Value	980	1,392
HTM securities
HTM securities, Amortized Cost	16	28
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
HTM securities, Fair Value	16	28
Non-agency MBS
AFS securities
AFS securities, Amortized Cost	351	384
Gross Unrealized Gains	203	192
Gross Unrealized Losses	0	0
AFS securities, Fair Value	554	576
Other
AFS securities
AFS securities, Amortized Cost	38	8
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
AFS securities, Fair Value	39	8
HTM securities
HTM securities, Amortized Cost	1	43
Gross Unrealized Gains	0	2
Gross Unrealized Losses	0	0
HTM securities, Fair Value	$ 1	$ 45

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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