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Goodwill and Other Intangibles - Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Thousands | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Goodwill And Other Intangible Assets [Line Items] |  |  |
Goodwill | $ 174,518 | $ 180,867 | $ 180,867
Sale of component of business segment | (6,349) |  |
Commercial Banking [Member] |  |  |
Goodwill And Other Intangible Assets [Line Items] |  |  |
Goodwill | 59,419 | 59,419 | 59,419
Institutional Banking [Member] |  |  |
Goodwill And Other Intangible Assets [Line Items] |  |  |
Goodwill | 51,332 | 51,332 | 51,332
Personal Banking [Member] |  |  |
Goodwill And Other Intangible Assets [Line Items] |  |  |
Goodwill | 63,767 | $ 70,116 | $ 70,116
Sale of component of business segment | $ (6,349) |  |

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