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Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | $ 5,407,376 | $ 4,879,347 | $ 4,683,750
Commercial Financial Agricultural And Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 1,163,383 | 1,139,547 |
Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 248,868 | 219,621 |
Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 1,426,370 | 1,229,192 |
Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 2,019,096 | 1,742,210 |
Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 549,659 | 548,777 |
Commercial Financial Agricultural And Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 1,163,383 | 1,139,547 | 1,150,906
Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 248,868 | 219,621 | 220,736
Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 1,426,370 | 1,229,192 | 1,224,465
Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 2,019,096 | 1,742,210 | 1,479,000
Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 549,659 | 548,777 | $ 608,643
Originated Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total past due and nonaccrual | 41,362 | 40,497 |
Current | 4,965,999 | 4,737,974 |
Loans | 5,007,361 | 4,778,471 |
Originated Loans [Member] | Commercial Financial Agricultural And Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total past due and nonaccrual | 19,220 | 20,554 |
Current | 1,103,521 | 1,110,594 |
Loans | 1,122,741 | 1,131,148 |
Originated Loans [Member] | Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total past due and nonaccrual | 199 | 183 |
Current | 242,706 | 217,657 |
Loans | 242,905 | 217,840 |
Originated Loans [Member] | Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total past due and nonaccrual | 12,944 | 12,009 |
Current | 1,193,175 | 1,153,842 |
Loans | 1,206,119 | 1,165,851 |
Originated Loans [Member] | Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total past due and nonaccrual | 5,472 | 4,092 |
Current | 1,886,713 | 1,712,951 |
Loans | 1,892,185 | 1,717,043 |
Originated Loans [Member] | Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total past due and nonaccrual | 3,527 | 3,659 |
Current | 539,884 | 542,930 |
Loans | 543,411 | 546,589 |
Originated Loans [Member] | Commercial Financial Agricultural And Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 1,122,741 | 1,131,148 |
Originated Loans [Member] | Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 242,905 | 217,840 |
Originated Loans [Member] | Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 1,206,119 | 1,165,851 |
Originated Loans [Member] | Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 1,892,185 | 1,717,043 |
Originated Loans [Member] | Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 543,411 | 546,589 |
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 9,292 | 8,601 |
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial Financial Agricultural And Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 378 | 2,380 |
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 199 | 183 |
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 4,618 | 4,133 |
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 2,198 | 265 |
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 1,899 | 1,640 |
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 1,883 | 2,363 |
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial Financial Agricultural And Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 61 | 171 |
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 0 | 0 |
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 1,025 | 1,089 |
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 28 | 327 |
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 769 | 776 |
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 1,745 | 2,097 |
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Financial Agricultural And Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 40 | 75 |
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 0 | 0 |
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 1,076 | 995 |
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 6 | 57 |
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 623 | 970 |
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 28,442 | 27,436 |
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial Financial Agricultural And Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 18,741 | 17,928 |
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 0 | 0 |
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 6,225 | 5,792 |
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 3,240 | 3,443 |
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 236 | 273 |
Acquired Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total past due and nonaccrual | 2,570 | 1,302 |
Current | 397,445 | 99,574 |
Loans | 400,015 | 100,876 |
Acquired Loans [Member] | Commercial Financial Agricultural And Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total past due and nonaccrual | 449 | 486 |
Current | 40,193 | 7,913 |
Loans | 40,642 | 8,399 |
Acquired Loans [Member] | Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total past due and nonaccrual | 0 | 0 |
Current | 5,963 | 1,781 |
Loans | 5,963 | 1,781 |
Acquired Loans [Member] | Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total past due and nonaccrual | 945 | 643 |
Current | 219,306 | 62,698 |
Loans | 220,251 | 63,341 |
Acquired Loans [Member] | Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total past due and nonaccrual | 1,077 | 162 |
Current | 125,834 | 25,005 |
Loans | 126,911 | 25,167 |
Acquired Loans [Member] | Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Total past due and nonaccrual | 99 | 11 |
Current | 6,149 | 2,177 |
Loans | 6,248 | 2,188 |
Acquired Loans [Member] | Commercial Financial Agricultural And Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 40,642 | 8,399 |
Acquired Loans [Member] | Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 5,963 | 1,781 |
Acquired Loans [Member] | Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 220,251 | 63,341 |
Acquired Loans [Member] | Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 126,911 | 25,167 |
Acquired Loans [Member] | Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans | 6,248 | 2,188 |
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 190 | 635 |
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial Financial Agricultural And Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 6 | 486 |
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 0 | 0 |
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 148 | 148 |
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 0 | 0 |
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 36 | 1 |
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 36 | 46 |
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial Financial Agricultural And Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 7 | 0 |
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 0 | 0 |
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 9 | 39 |
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 0 | 0 |
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 20 | 7 |
Acquired Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 109 | 34 |
Acquired Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Financial Agricultural And Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 0 | 0 |
Acquired Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 0 | 0 |
Acquired Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 83 | 34 |
Acquired Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 0 | 0 |
Acquired Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 26 | 0 |
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 2,235 | 587 |
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial Financial Agricultural And Other [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 436 | 0 |
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Real Estate Construction [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 0 | 0 |
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Residential real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 705 | 422 |
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial real estate [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | 1,077 | 162 |
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Loans To Individuals [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Financing Receivable, Recorded Investment, Past Due | $ 17 | $ 3 |

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