Investment Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
AFS, Amortized Cost
Due in one year or less	$ 1,414
Due after one year through five years	2,718
Due after five years through ten years	688
Due after ten years	70,340
Debt Securities, Available-for-sale, Amortized Cost, Total	75,160	$ 74,227
AFS, Fair Value
Due in one year or less	1,427
Due after one year through five years	2,834
Due after five years through ten years	722
Due after ten years	72,822
Debt Securities, Available-for-sale, Total	77,805	74,727
U.S. Treasury
AFS, Amortized Cost
Due in one year or less	1,353
Due after one year through five years	851
Due after five years through ten years	16
Due after ten years	0
Debt Securities, Available-for-sale, Amortized Cost, Total	2,220	2,275
AFS, Fair Value
Due in one year or less	1,365
Due after one year through five years	878
Due after five years through ten years	17
Due after ten years	0
Debt Securities, Available-for-sale, Total	2,260	2,276
US Government-sponsored Enterprises Debt Securities
AFS, Amortized Cost
Due in one year or less	13
Due after one year through five years	1,756
Due after five years through ten years	0
Due after ten years	75
Debt Securities, Available-for-sale, Amortized Cost, Total	1,844	1,847
AFS, Fair Value
Due in one year or less	13
Due after one year through five years	1,842
Due after five years through ten years	0
Due after ten years	80
Debt Securities, Available-for-sale, Total	1,935	1,881
States and political subdivisions
AFS, Amortized Cost
Due in one year or less	47
Due after one year through five years	102
Due after five years through ten years	142
Due after ten years	208
Debt Securities, Available-for-sale, Amortized Cost, Total	499	557
AFS, Fair Value
Due in one year or less	48
Due after one year through five years	105
Due after five years through ten years	156
Due after ten years	228
Debt Securities, Available-for-sale, Total	537	585
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	519
Due after ten years	68,621
Debt Securities, Available-for-sale, Amortized Cost, Total	69,141	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	538
Due after ten years	71,008
Debt Securities, Available-for-sale, Total	71,547	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	10
Due after ten years	1,408
Debt Securities, Available-for-sale, Amortized Cost, Total	1,420	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	1,479
Debt Securities, Available-for-sale, Total	1,491	1,341
Other
AFS, Amortized Cost
Due in one year or less	1
Due after one year through five years	6
Due after five years through ten years	1
Due after ten years	28
Debt Securities, Available-for-sale, Amortized Cost, Total	36	40
AFS, Fair Value
Due in one year or less	1
Due after one year through five years	6
Due after five years through ten years	1
Due after ten years	27
Debt Securities, Available-for-sale, Total	$ 35	$ 40