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Loans and Allowance for Credit Losses - Recorded Investment and Unpaid Principal Balance for Impaired Loans with Associated Allowance (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | $ 54,053 |  |
Unpaid Principal Balance | 65,331 |  |
Related Allowance | 4,906 |  |
Average Recorded Investment | 50,728 |  |
Interest Income Recognized | 639 |  |
Commercial Financial Agricultural And Other With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 2,025 |  |
Unpaid Principal Balance | 2,725 |  |
Average Recorded Investment | 6,371 |  |
Interest Income Recognized | 80 |  |
Commercial Financial Agricultural And Other With No Related Allowance [Member] | Time and demand loans |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 2,025 |  |
Unpaid Principal Balance | 2,725 |  |
Average Recorded Investment | 6,371 |  |
Interest Income Recognized | 80 |  |
Real Estate Construction With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 54 |  |
Unpaid Principal Balance | 53 |  |
Average Recorded Investment | 146 |  |
Interest Income Recognized | 14 |  |
Residential real estate with no related allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 10,939 |  |
Unpaid Principal Balance | 13,258 |  |
Average Recorded Investment | 11,913 |  |
Interest Income Recognized | 335 |  |
Residential real estate with no related allowance recorded [Member] | Senior Lien |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 6,062 |  |
Unpaid Principal Balance | 7,575 |  |
Average Recorded Investment | 6,605 |  |
Interest Income Recognized | 241 |  |
Residential real estate with no related allowance recorded [Member] | Junior Lien |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 4,877 |  |
Unpaid Principal Balance | 5,683 |  |
Average Recorded Investment | 5,308 |  |
Interest Income Recognized | 94 |  |
Commercial Real Estate With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 20,650 |  |
Unpaid Principal Balance | 23,641 |  |
Average Recorded Investment | 22,287 |  |
Interest Income Recognized | 184 |  |
Commercial Real Estate With No Related Allowance [Member] | Multifamily |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 1 |  |
Unpaid Principal Balance | 82 |  |
Average Recorded Investment | 1 |  |
Interest Income Recognized | 0 |  |
Commercial Real Estate With No Related Allowance [Member] | Nonowner Occupied Commercial Real Estate |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 16,786 |  |
Unpaid Principal Balance | 19,459 |  |
Average Recorded Investment | 18,536 |  |
Interest Income Recognized | 82 |  |
Commercial Real Estate With No Related Allowance [Member] | Owner Occupied Commercial Real Estate |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 3,863 |  |
Unpaid Principal Balance | 4,100 |  |
Average Recorded Investment | 3,750 |  |
Interest Income Recognized | 102 |  |
Loans To Individuals With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 418 |  |
Unpaid Principal Balance | 447 |  |
Average Recorded Investment | 470 |  |
Interest Income Recognized | 13 |  |
Loans To Individuals With No Related Allowance [Member] | Automobiles |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 405 |  |
Unpaid Principal Balance | 430 |  |
Average Recorded Investment | 447 |  |
Interest Income Recognized | 13 |  |
Loans To Individuals With No Related Allowance [Member] | Other consumer loans |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 13 |  |
Unpaid Principal Balance | 17 |  |
Average Recorded Investment | 23 |  |
Interest Income Recognized | 0 |  |
Subtotal with no related allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 34,086 |  |
Unpaid Principal Balance | 40,124 |  |
Average Recorded Investment | 41,187 |  |
Interest Income Recognized | 626 |  |
Commercial, financial, agricultural and other With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 4,210 |  |
Unpaid Principal Balance | 9,377 |  |
Related Allowance | 1,268 |  |
Average Recorded Investment | 1,544 |  |
Interest Income Recognized | 3 |  |
Commercial, financial, agricultural and other With an allowance recorded [Member] | Time and demand loans |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 4,210 |  |
Unpaid Principal Balance | 9,377 |  |
Related Allowance | 1,268 |  |
Average Recorded Investment | 1,544 |  |
Interest Income Recognized | 3 |  |
Real estate construction with an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 0 |  |
Unpaid Principal Balance | 0 |  |
Related Allowance | 0 |  |
Average Recorded Investment | 0 |  |
Interest Income Recognized | 0 |  |
Residential real estate With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 0 |  |
Unpaid Principal Balance | 0 |  |
Related Allowance | 0 |  |
Average Recorded Investment | 0 |  |
Interest Income Recognized | 0 |  |
Residential real estate With an allowance recorded [Member] | Senior Lien |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 0 |  |
Unpaid Principal Balance | 0 |  |
Related Allowance | 0 |  |
Average Recorded Investment | 0 |  |
Interest Income Recognized | 0 |  |
Residential real estate With an allowance recorded [Member] | Junior Lien |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 0 |  |
Unpaid Principal Balance | 0 |  |
Related Allowance | 0 |  |
Average Recorded Investment | 0 |  |
Interest Income Recognized | 0 |  |
Commercial real estate With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 15,757 |  |
Unpaid Principal Balance | 15,830 |  |
Related Allowance | 3,638 |  |
Average Recorded Investment | 7,997 |  |
Interest Income Recognized | 10 |  |
Commercial real estate With an allowance recorded [Member] | Multifamily |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 459 |  |
Unpaid Principal Balance | 470 |  |
Related Allowance | 116 |  |
Average Recorded Investment | 395 |  |
Interest Income Recognized | 0 |  |
Commercial real estate With an allowance recorded [Member] | Nonowner Occupied Commercial Real Estate |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 15,060 |  |
Unpaid Principal Balance | 15,122 |  |
Related Allowance | 3,508 |  |
Average Recorded Investment | 7,363 |  |
Interest Income Recognized | 0 |  |
Commercial real estate With an allowance recorded [Member] | Owner Occupied Commercial Real Estate |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 238 |  |
Unpaid Principal Balance | 238 |  |
Related Allowance | 14 |  |
Average Recorded Investment | 239 |  |
Interest Income Recognized | 10 |  |
Loans to individuals With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 0 |  |
Unpaid Principal Balance | 0 |  |
Related Allowance | 0 |  |
Average Recorded Investment | 0 |  |
Interest Income Recognized | 0 |  |
Loans to individuals With an allowance recorded [Member] | Automobiles |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 0 |  |
Unpaid Principal Balance | 0 |  |
Related Allowance | 0 |  |
Average Recorded Investment | 0 |  |
Interest Income Recognized | 0 |  |
Loans to individuals With an allowance recorded [Member] | Other consumer loans |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 0 |  |
Unpaid Principal Balance | 0 |  |
Related Allowance | 0 |  |
Average Recorded Investment | 0 |  |
Interest Income Recognized | 0 |  |
Subtotal With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 19,967 |  |
Unpaid Principal Balance | 25,207 |  |
Related Allowance | 4,906 |  |
Average Recorded Investment | 9,541 |  |
Interest Income Recognized | $ 13 |  |
Originated Loans [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | $ 29,698 |
Unpaid Principal Balance |  | 39,122 |
Related Allowance |  | 2,432 |
Average Recorded Investment |  | 27,154 | $ 41,386
Interest Income Recognized |  | 684 | 1,098
Originated Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 1,848 |
Unpaid Principal Balance |  | 6,997 |
Average Recorded Investment |  | 2,411 | 18,480
Interest Income Recognized |  | 66 | 602
Originated Loans [Member] | Real Estate Construction With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 0 |
Unpaid Principal Balance |  | 0 |
Average Recorded Investment |  | 0 | 0
Interest Income Recognized |  | 0 | 0
Originated Loans [Member] | Residential real estate with no related allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 10,372 |
Unpaid Principal Balance |  | 12,437 |
Average Recorded Investment |  | 10,819 | 10,651
Interest Income Recognized |  | 365 | 271
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 3,015 |
Unpaid Principal Balance |  | 3,210 |
Average Recorded Investment |  | 7,455 | 7,919
Interest Income Recognized |  | 156 | 177
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 406 |
Unpaid Principal Balance |  | 640 |
Average Recorded Investment |  | 371 | 310
Interest Income Recognized |  | 17 | 11
Originated Loans [Member] | Subtotal with no related allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 15,641 |
Unpaid Principal Balance |  | 23,284 |
Average Recorded Investment |  | 21,056 | 37,360
Interest Income Recognized |  | 604 | 1,061
Originated Loans [Member] | Commercial, financial, agricultural and other With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 8,290 |
Unpaid Principal Balance |  | 10,032 |
Related Allowance |  | 1,580 |
Average Recorded Investment |  | 4,110 | 2,531
Interest Income Recognized |  | 77 | 20
Originated Loans [Member] | Real estate construction with an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 0 |
Unpaid Principal Balance |  | 0 |
Related Allowance |  | 0 |
Average Recorded Investment |  | 0 | 0
Interest Income Recognized |  | 0 | 0
Originated Loans [Member] | Residential real estate With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 474 |
Unpaid Principal Balance |  | 498 |
Related Allowance |  | 1 |
Average Recorded Investment |  | 241 | 504
Interest Income Recognized |  | 0 | 13
Originated Loans [Member] | Commercial real estate With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 5,293 |
Unpaid Principal Balance |  | 5,308 |
Related Allowance |  | 851 |
Average Recorded Investment |  | 1,747 | 991
Interest Income Recognized |  | 3 | 4
Originated Loans [Member] | Loans to individuals With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 0 |
Unpaid Principal Balance |  | 0 |
Related Allowance |  | 0 |
Average Recorded Investment |  | 0 | 0
Interest Income Recognized |  | 0 | 0
Originated Loans [Member] | Subtotal With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 14,057 |
Unpaid Principal Balance |  | 15,838 |
Related Allowance |  | 2,432 |
Average Recorded Investment |  | 6,098 | 4,026
Interest Income Recognized |  | 80 | 37
Acquired Loans [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 2,519 |
Unpaid Principal Balance |  | 2,993 |
Related Allowance |  | 0 |
Average Recorded Investment |  | 5,225 | 3,712
Interest Income Recognized |  | 26 | 15
Acquired Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 73 |
Unpaid Principal Balance |  | 73 |
Average Recorded Investment |  | 2,479 | 214
Interest Income Recognized |  | 0 | 10
Acquired Loans [Member] | Real Estate Construction With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 0 |
Unpaid Principal Balance |  | 0 |
Average Recorded Investment |  | 0 | 0
Interest Income Recognized |  | 0 | 0
Acquired Loans [Member] | Residential real estate with no related allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 2,136 |
Unpaid Principal Balance |  | 2,585 |
Average Recorded Investment |  | 1,986 | 1,906
Interest Income Recognized |  | 8 | 5
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 298 |
Unpaid Principal Balance |  | 320 |
Average Recorded Investment |  | 747 | 1,565
Interest Income Recognized |  | 18 | 0
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 12 |
Unpaid Principal Balance |  | 15 |
Average Recorded Investment |  | 13 | 16
Interest Income Recognized |  | 0 | 0
Acquired Loans [Member] | Subtotal with no related allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 2,519 |
Unpaid Principal Balance |  | 2,993 |
Average Recorded Investment |  | 5,225 | 3,701
Interest Income Recognized |  | 26 | 15
Acquired Loans [Member] | Commercial, financial, agricultural and other With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 0 |
Unpaid Principal Balance |  | 0 |
Related Allowance |  | 0 |
Average Recorded Investment |  | 0 | 11
Interest Income Recognized |  | 0 | 0
Acquired Loans [Member] | Real estate construction with an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 0 |
Unpaid Principal Balance |  | 0 |
Related Allowance |  | 0 |
Average Recorded Investment |  | 0 | 0
Interest Income Recognized |  | 0 | 0
Acquired Loans [Member] | Residential real estate With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 0 |
Unpaid Principal Balance |  | 0 |
Related Allowance |  | 0 |
Average Recorded Investment |  | 0 | 0
Interest Income Recognized |  | 0 | 0
Acquired Loans [Member] | Commercial real estate With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 0 |
Unpaid Principal Balance |  | 0 |
Related Allowance |  | 0 |
Average Recorded Investment |  | 0 | 0
Interest Income Recognized |  | 0 | 0
Acquired Loans [Member] | Loans to individuals With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 0 |
Unpaid Principal Balance |  | 0 |
Related Allowance |  | 0 |
Average Recorded Investment |  | 0 | 0
Interest Income Recognized |  | 0 | 0
Acquired Loans [Member] | Subtotal With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment |  | 0 |
Unpaid Principal Balance |  | 0 |
Related Allowance |  | 0 |
Average Recorded Investment |  | 0 | 11
Interest Income Recognized |  | $ 0 | $ 0

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