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Investment securities - Investment Securities in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024
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Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
AFS, less than 12 months, fair value | $ 594 | $ 8,966
AFS, less than 12 months, gross unrealized losses | 2 | 107
AFS, 12 months or longer, fair value | 2,012 | 4,243
AFS, 12 months or longer, gross unrealized losses | 71 | 132
Total investment securities, less than 12 months, fair value | 688 | 10,051
Total investment securities, less than 12 months, gross unrealized losses | 3 | 127
Total investment securities, 12 months or longer, fair value | 12,822 | 16,013
Total investment securities, 12 months or longer, gross unrealized losses | 876 | 1,361
Schedule of Held-to-Maturity Securities [Line Items] |  |
HTM, less than 12 months, fair value | 94 | 1,085
HTM, Less than 12 months, gross unrealized losses | 1 | 20
HTM, 12 months or longer, fair value | 10,810 | 11,770
HTM, 12 months or longer, gross unrealized losses | 805 | 1,229
U.S. Treasury |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
AFS, less than 12 months, fair value | 0 | 1,971
AFS, less than 12 months, gross unrealized losses | 0 | 9
AFS, 12 months or longer, fair value | 247 | 2,554
AFS, 12 months or longer, gross unrealized losses | 3 | 18
Schedule of Held-to-Maturity Securities [Line Items] |  |
HTM, less than 12 months, fair value | 0 | 50
HTM, Less than 12 months, gross unrealized losses | 0 | 0
HTM, 12 months or longer, fair value | 438 | 951
HTM, 12 months or longer, gross unrealized losses | 6 | 14
Commercial |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
AFS, less than 12 months, fair value | 135 | 2,566
AFS, less than 12 months, gross unrealized losses | 0 | 45
AFS, 12 months or longer, fair value | 323 | 64
AFS, 12 months or longer, gross unrealized losses | 2 | 0
Schedule of Held-to-Maturity Securities [Line Items] |  |
HTM, less than 12 months, fair value | 0 | 0
HTM, Less than 12 months, gross unrealized losses | 0 | 0
HTM, 12 months or longer, fair value | 1,943 | 1,877
HTM, 12 months or longer, gross unrealized losses | 77 | 157
Residential |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
AFS, less than 12 months, fair value | 458 | 4,429
AFS, less than 12 months, gross unrealized losses | 2 | 53
AFS, 12 months or longer, fair value | 1,442 | 1,623
AFS, 12 months or longer, gross unrealized losses | 66 | 114
Schedule of Held-to-Maturity Securities [Line Items] |  |
HTM, less than 12 months, fair value | 83 | 996
HTM, Less than 12 months, gross unrealized losses | 1 | 19
HTM, 12 months or longer, fair value | 6,513 | 6,811
HTM, 12 months or longer, gross unrealized losses | 661 | 942
Other |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract] |  |
AFS, less than 12 months, fair value | 1 | 0
AFS, less than 12 months, gross unrealized losses | 0 | 0
AFS, 12 months or longer, fair value | 0 | 2
AFS, 12 months or longer, gross unrealized losses | 0 | 0
State and political subdivisions |  |
Schedule of Held-to-Maturity Securities [Line Items] |  |
HTM, less than 12 months, fair value | 9 | 39
HTM, Less than 12 months, gross unrealized losses | 0 | 1
HTM, 12 months or longer, fair value | 1,916 | 2,131
HTM, 12 months or longer, gross unrealized losses | 61 | $ 116
Privately issued |  |
Schedule of Held-to-Maturity Securities [Line Items] |  |
HTM, less than 12 months, fair value | 2 |
HTM, Less than 12 months, gross unrealized losses | 0 |
HTM, 12 months or longer, fair value | 0 |
HTM, 12 months or longer, gross unrealized losses | $ 0 |

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