Investment Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
AFS, Amortized Cost
Due in one year or less	$ 4,086
Due after one year through five years	9,458
Due after five years through ten years	654
Due after ten years	54,318
Debt Securities, Available-for-sale, Amortized Cost, Total	68,516	$ 78,849
AFS, Fair Value
Due in one year or less	4,101
Due after one year through five years	9,595
Due after five years through ten years	639
Due after ten years	49,776
Debt Securities, Available-for-sale, Total	64,111	67,366
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date
Debt Securities, Held-to-Maturity, Amortized Cost, Total	51,495	54,107
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Fair Value
Debt Securities, Held-to-Maturity, Fair Value	43,229	44,630
U.S. Treasury
AFS, Amortized Cost
Due in one year or less	4,019
Due after one year through five years	9,272
Due after five years through ten years	8
Due after ten years	29
Debt Securities, Available-for-sale, Amortized Cost, Total	13,328	10,511
AFS, Fair Value
Due in one year or less	4,034
Due after one year through five years	9,408
Due after five years through ten years	8
Due after ten years	25
Debt Securities, Available-for-sale, Total	13,475	10,041
GSE
AFS, Amortized Cost
Due in one year or less	2
Due after one year through five years	5
Due after five years through ten years	12
Due after ten years	368
Debt Securities, Available-for-sale, Amortized Cost, Total	387	393
AFS, Fair Value
Due in one year or less	2
Due after one year through five years	5
Due after five years through ten years	12
Due after ten years	346
Debt Securities, Available-for-sale, Total	365	362
Agency MBS – residential
AFS, Amortized Cost
Due in one year or less	0
Due after one year through five years	97
Due after five years through ten years	407
Due after ten years	50,948
Debt Securities, Available-for-sale, Amortized Cost, Total	51,452	60,989
AFS, Fair Value
Due in one year or less	0
Due after one year through five years	95
Due after five years through ten years	392
Due after ten years	46,979
Debt Securities, Available-for-sale, Total	47,466	51,289
Debt Securities, Held-to-Maturity, Amortized Cost, after Allowance for Credit Loss, Maturity, Allocated and Single Maturity Date
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	51,495
Debt Securities, Held-to-Maturity, Amortized Cost, Total	51,495	54,107
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, 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	43,229
Debt Securities, Held-to-Maturity, Fair Value	43,229	44,630
Agency MBS – commercial
AFS, Amortized Cost
Due in one year or less	0
Due after one year through five years	9
Due after five years through ten years	71
Due after ten years	2,837
Debt Securities, Available-for-sale, Amortized Cost, Total	2,917	2,817
AFS, Fair Value
Due in one year or less	0
Due after one year through five years	9
Due after five years through ten years	68
Due after ten years	2,294
Debt Securities, Available-for-sale, Total	2,371	2,248
States and political subdivisions
AFS, Amortized Cost
Due in one year or less	65
Due after one year through five years	68
Due after five years through ten years	147
Due after ten years	136
Debt Securities, Available-for-sale, Amortized Cost, Total	416	421
AFS, Fair Value
Due in one year or less	65
Due after one year through five years	71
Due after five years through ten years	150
Due after ten years	132
Debt Securities, Available-for-sale, Total	418	425
Non-agency MBS
AFS, Amortized Cost
Debt Securities, Available-for-sale, Amortized Cost, Total		3,698
AFS, Fair Value
Debt Securities, Available-for-sale, Total		2,981
Other
AFS, Amortized Cost
Due in one year or less	0
Due after one year through five years	7
Due after five years through ten years	9
Due after ten years	0
Debt Securities, Available-for-sale, Amortized Cost, Total	16	20
AFS, Fair Value
Due in one year or less	0
Due after one year through five years	7
Due after five years through ten years	9
Due after ten years	0
Debt Securities, Available-for-sale, Total	$ 16	$ 20