Securities - Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
AFS securities
AFS securities, Amortized Cost	$ 25,602	$ 25,689
Gross Unrealized Gains	394	247
Gross Unrealized Losses	194	898
AFS securities, Fair Value	25,802	25,038
HTM securities
HTM securities, Amortized Cost	19,487	20,552
Gross Unrealized Gains	134	31
Gross Unrealized Losses	56	536
HTM securities, Fair Value	19,565	20,047
U.S. Treasury
AFS securities
AFS securities, Amortized Cost	1,120	3,503
Gross Unrealized Gains	1	22
Gross Unrealized Losses	9	84
AFS securities, Fair Value	1,112	3,441
HTM securities
HTM securities, Amortized Cost	1,099	1,099
Gross Unrealized Gains	6	0
Gross Unrealized Losses	0	6
HTM securities, Fair Value	1,105	1,093
GSE
AFS securities
AFS securities, Amortized Cost	246	209
Gross Unrealized Gains	3	0
Gross Unrealized Losses	2	9
AFS securities, Fair Value	247	200
HTM securities
HTM securities, Amortized Cost	2,199	2,199
Gross Unrealized Gains	27	4
Gross Unrealized Losses	0	43
HTM securities, Fair Value	2,226	2,160
Agency MBS
AFS securities
AFS securities, Amortized Cost	23,428	20,927
Gross Unrealized Gains	184	15
Gross Unrealized Losses	171	787
AFS securities, Fair Value	23,441	20,155
HTM securities
HTM securities, Amortized Cost	16,186	17,248
Gross Unrealized Gains	101	27
Gross Unrealized Losses	56	487
HTM securities, Fair Value	16,231	16,788
States and political subdivisions
AFS securities
AFS securities, Amortized Cost	568	694
Gross Unrealized Gains	31	25
Gross Unrealized Losses	12	18
AFS securities, Fair Value	587	701
HTM securities
HTM securities, Amortized Cost	3	5
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
HTM securities, Fair Value	3	5
Non-agency MBS
AFS securities
AFS securities, Amortized Cost	207	321
Gross Unrealized Gains	175	184
Gross Unrealized Losses	0	0
AFS securities, Fair Value	382	505
Other
AFS securities
AFS securities, Amortized Cost	33	35
Gross Unrealized Gains	0	1
Gross Unrealized Losses	0	0
AFS securities, Fair Value	33	36
HTM securities
HTM securities, Amortized Cost	0	1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
HTM securities, Fair Value	$ 0	$ 1

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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