Investment Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
AFS, Amortized Cost
Due in one year or less	$ 1,422
Due after one year through five years	2,768
Due after five years through ten years	732
Due after ten years	70,728
Debt Securities, Available-for-sale, Amortized Cost	75,650	$ 74,227
Debt Securities, Available-for-sale, Amortized Cost, Total	75,650	74,227
AFS, Fair Value
Due in one year or less	1,440
Due after one year through five years	2,881
Due after five years through ten years	761
Due after ten years	73,316
AFS securities, Fair Value	78,398	74,727
Debt Securities, Available-for-sale, Total	78,398	74,727
U.S. Treasury
AFS, Amortized Cost
Due in one year or less	1,381
Due after one year through five years	876
Due after five years through ten years	15
Due after ten years	0
Debt Securities, Available-for-sale, Amortized Cost	2,272	2,275
Debt Securities, Available-for-sale, Amortized Cost, Total	2,272	2,275
AFS, Fair Value
Due in one year or less	1,399
Due after one year through five years	903
Due after five years through ten years	17
Due after ten years	0
AFS securities, Fair Value	2,319	2,276
Debt Securities, Available-for-sale, Total	2,319	2,276
US Government-sponsored Enterprises Debt Securities [Member]
AFS, Amortized Cost
Due in one year or less	0
Due after one year through five years	1,769
Due after five years through ten years	0
Due after ten years	77
Debt Securities, Available-for-sale, Amortized Cost	1,846	1,847
Debt Securities, Available-for-sale, Amortized Cost, Total	1,846	1,847
AFS, Fair Value
Due in one year or less	0
Due after one year through five years	1,853
Due after five years through ten years	0
Due after ten years	80
AFS securities, Fair Value	1,933	1,881
Debt Securities, Available-for-sale, Total	1,933	1,881
States and political subdivisions
AFS, Amortized Cost
Due in one year or less	39
Due after one year through five years	114
Due after five years through ten years	151
Due after ten years	209
Debt Securities, Available-for-sale, Amortized Cost	513	557
Debt Securities, Available-for-sale, Amortized Cost, Total	513	557
AFS, Fair Value
Due in one year or less	39
Due after one year through five years	116
Due after five years through ten years	163
Due after ten years	221
AFS securities, Fair Value	539	585
Debt Securities, Available-for-sale, Total	539	585
Non-agency MBS
AFS, Amortized Cost
Due in one year or less	0
Due after one year through five years	0
Due after five years through ten years	0
Due after ten years	185
Debt Securities, Available-for-sale, Amortized Cost	185	190
Debt Securities, Available-for-sale, Amortized Cost, Total	185	190
AFS, Fair Value
Due in one year or less	0
Due after one year through five years	0
Due after five years through ten years	0
Due after ten years	298
AFS securities, Fair Value	298	368
Debt Securities, Available-for-sale, Total	298	368
Agency MBS | Residential Mortgage Backed Securities
AFS, Amortized Cost
Due in one year or less	0
Due after one year through five years	1
Due after five years through ten years	556
Due after ten years	68,750
Debt Securities, Available-for-sale, Amortized Cost	69,307	67,983
Debt Securities, Available-for-sale, Amortized Cost, Total	69,307	67,983
AFS, Fair Value
Due in one year or less	0
Due after one year through five years	1
Due after five years through ten years	570
Due after ten years	71,182
AFS securities, Fair Value	71,753	68,236
Debt Securities, Available-for-sale, Total	71,753	68,236
Agency MBS | Commercial Mortgage Backed Securities
AFS, Amortized Cost
Due in one year or less	0
Due after one year through five years	2
Due after five years through ten years	9
Due after ten years	1,478
Debt Securities, Available-for-sale, Amortized Cost	1,489	1,335
Debt Securities, Available-for-sale, Amortized Cost, Total	1,489	1,335
AFS, Fair Value
Due in one year or less	0
Due after one year through five years	2
Due after five years through ten years	10
Due after ten years	1,507
AFS securities, Fair Value	1,519	1,341
Debt Securities, Available-for-sale, Total	1,519	1,341
Other
AFS, Amortized Cost
Due in one year or less	2
Due after one year through five years	6
Due after five years through ten years	1
Due after ten years	29
Debt Securities, Available-for-sale, Amortized Cost	38	40
Debt Securities, Available-for-sale, Amortized Cost, Total	38	40
AFS, Fair Value
Due in one year or less	2
Due after one year through five years	6
Due after five years through ten years	1
Due after ten years	28
AFS securities, Fair Value	37	40
Debt Securities, Available-for-sale, Total	$ 37	$ 40