Investment Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
AFS, Amortized Cost
Due in one year or less	$ 1,397
Due after one year through five years	2,774
Due after five years through ten years	793
Due after ten years	69,263
Total	74,227	$ 25,689
Debt Securities, Available-for-sale, Amortized Cost, Total	74,227	25,689
AFS, Fair Value
Due in one year or less	1,404
Due after one year through five years	2,802
Due after five years through ten years	807
Due after ten years	69,714
AFS securities, Fair Value	74,727	25,038
Debt Securities, Available-for-sale, Total	74,727	25,038
U.S. Treasury
AFS, Amortized Cost
Due in one year or less	1,360
Due after one year through five years	899
Due after five years through ten years	16
Due after ten years	0
Total	2,275	3,503
Debt Securities, Available-for-sale, Amortized Cost, Total	2,275	3,503
AFS, Fair Value
Due in one year or less	1,367
Due after one year through five years	893
Due after five years through ten years	16
Due after ten years	0
AFS securities, Fair Value	2,276	3,441
Debt Securities, Available-for-sale, Total	2,276	3,441
US Government-sponsored Enterprises Debt Securities [Member]
AFS, Amortized Cost
Due in one year or less	0
Due after one year through five years	1,768
Due after five years through ten years	0
Due after ten years	79
Total	1,847	209
Debt Securities, Available-for-sale, Amortized Cost, Total	1,847	209
AFS, Fair Value
Due in one year or less	0
Due after one year through five years	1,800
Due after five years through ten years	0
Due after ten years	81
AFS securities, Fair Value	1,881	200
Debt Securities, Available-for-sale, Total	1,881	200
States and political subdivisions
AFS, Amortized Cost
Due in one year or less	35
Due after one year through five years	97
Due after five years through ten years	215
Due after ten years	210
Total	557	694
Debt Securities, Available-for-sale, Amortized Cost, Total	557	694
AFS, Fair Value
Due in one year or less	35
Due after one year through five years	100
Due after five years through ten years	226
Due after ten years	224
AFS securities, Fair Value	585	701
Debt Securities, Available-for-sale, Total	585	701
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	190
Total	190	321
Debt Securities, Available-for-sale, Amortized Cost, Total	190	321
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	368
AFS securities, Fair Value	368	505
Debt Securities, Available-for-sale, Total	368	505
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	552
Due after ten years	67,430
Total	67,983	20,170
Debt Securities, Available-for-sale, Amortized Cost, Total	67,983	20,170
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	554
Due after ten years	67,681
AFS securities, Fair Value	68,236	19,426
Debt Securities, Available-for-sale, Total	68,236	19,426
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	1,324
Total	1,335	757
Debt Securities, Available-for-sale, Amortized Cost, Total	1,335	757
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	10
Due after ten years	1,330
AFS securities, Fair Value	1,341	729
Debt Securities, Available-for-sale, Total	1,341	729
Other
AFS, Amortized Cost
Due in one year or less	2
Due after one year through five years	7
Due after five years through ten years	1
Due after ten years	30
Total	40	35
Debt Securities, Available-for-sale, Amortized Cost, Total	40	35
AFS, Fair Value
Due in one year or less	2
Due after one year through five years	7
Due after five years through ten years	1
Due after ten years	30
AFS securities, Fair Value	40	36
Debt Securities, Available-for-sale, Total	$ 40	$ 36