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Intangible Assets and Goodwill - Additional Information (Detail) - USD ($) |  |  | 12 Months Ended |
--- | --- | --- | --- | ---
Mar. 05, 2020 | Aug. 15, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items] |  |  |  |  |
Weighted-average life |  |  | 7 years 2 months 12 days |  |
Goodwill |  |  | $ 149,922,000 | $ 148,604,000 | $ 79,749,000
Pegasus Bank [Member] |  |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |  |
Date of acquisition |  | Aug. 15, 2019 |  |  |
Goodwill |  | $ 68,900,000 | $ 68,855,000 | 68,855,000 |
Citizens Bankshares, Inc [Member] |  |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |  |
Date of acquisition | Mar. 05, 2020 |  |  |  |
Goodwill | $ 1,300,000 |  |  |  |
Customer Related Intangibles [Member] |  |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |  |
Weighted-average life |  |  | 4 years |  |
Customer Related Intangibles [Member] | BancFirst Insurance Services, Inc. [Member] |  |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |  |
Impairment of intangible assets |  |  |  | $ 353,000 |
Core Deposit Intangibles [Member] |  |  |  |  |
Finite-Lived Intangible Assets [Line Items] |  |  |  |  |
Weighted-average life |  |  | 7 years 3 months 18 days |  |

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