Securities - Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
AFS securities
AFS securities, Amortized Cost	$ 25,433	$ 25,009
Gross Unrealized Gains	222	237
Gross Unrealized Losses	1,369	699
AFS securities, Fair Value	24,286	24,547
HTM securities
HTM securities, Amortized Cost	21,082	23,027
Gross Unrealized Gains	31	54
Gross Unrealized Losses	850	244
HTM securities, Fair Value	20,263	22,837
U.S. Treasury
AFS securities
AFS securities, Amortized Cost	2,505	2,368
Gross Unrealized Gains	0	0
Gross Unrealized Losses	127	77
AFS securities, Fair Value	2,378	2,291
HTM securities
HTM securities, Amortized Cost	1,099	1,098
Gross Unrealized Gains	0	8
Gross Unrealized Losses	15	0
HTM securities, Fair Value	1,084	1,106
GSE
AFS securities
AFS securities, Amortized Cost	210	187
Gross Unrealized Gains	0	0
Gross Unrealized Losses	12	8
AFS securities, Fair Value	198	179
HTM securities
HTM securities, Amortized Cost	2,199	2,198
Gross Unrealized Gains	0	11
Gross Unrealized Losses	71	22
HTM securities, Fair Value	2,128	2,187
Agency MBS
AFS securities
AFS securities, Amortized Cost	21,582	20,683
Gross Unrealized Gains	3	8
Gross Unrealized Losses	1,213	590
AFS securities, Fair Value	20,372	20,101
HTM securities
HTM securities, Amortized Cost	17,775	19,660
Gross Unrealized Gains	31	33
Gross Unrealized Losses	764	222
HTM securities, Fair Value	17,042	19,471
States and political subdivisions
AFS securities
AFS securities, Amortized Cost	764	1,379
Gross Unrealized Gains	24	37
Gross Unrealized Losses	17	24
AFS securities, Fair Value	771	1,392
HTM securities
HTM securities, Amortized Cost	8	28
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
HTM securities, Fair Value	8	28
Non-agency MBS
AFS securities
AFS securities, Amortized Cost	335	384
Gross Unrealized Gains	194	192
Gross Unrealized Losses	0	0
AFS securities, Fair Value	529	576
Other
AFS securities
AFS securities, Amortized Cost	37	8
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
AFS securities, Fair Value	38	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 amortized cost (held-to-maturity).

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