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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 | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Acquired Loans [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | $ 2,224 | $ 468 |
Impaired Financing Receivable, Interest Income, Accrual Method | 1 | 0 |
Related allowance | 12 |  | $ 33
Recorded investment | 2,198 |  | 2,289
Unpaid principal balance | 3,501 |  | 3,545
Acquired Loans [Member] | Commercial Loan [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 |
Related allowance | 0 |  | 0
Recorded investment | 0 |  | 0
Unpaid principal balance | 0 |  | 0
Acquired Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 411 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 |
Recorded investment | 411 |  | 436
Unpaid principal balance | 424 |  | 449
Acquired Loans [Member] | Real Estate Construction With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 |
Recorded investment | 0 |  | 0
Unpaid principal balance | 0 |  | 0
Acquired Loans [Member] | Residential Real Estate With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 678 | 241 |
Impaired Financing Receivable, Interest Income, Accrual Method | 1 | 0 |
Recorded investment | 677 |  | 666
Unpaid principal balance | 1,003 |  | 965
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 907 | 162 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 |
Recorded investment | 891 |  | 940
Unpaid principal balance | 1,811 |  | 1,842
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 17 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 |
Recorded investment | 17 |  | 17
Unpaid principal balance | 17 |  | 17
Acquired Loans [Member] | With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 2,013 | 403 |
Impaired Financing Receivable, Interest Income, Accrual Method | 1 | 0 |
Recorded investment | 1,996 |  | 2,059
Unpaid principal balance | 3,255 |  | 3,273
Acquired Loans [Member] | Real Estate Construction With Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 |
Related allowance | 0 |  | 0
Recorded investment | 0 |  | 0
Unpaid principal balance | 0 |  | 0
Acquired Loans [Member] | Residential Real Estate With Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 93 | 65 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 |
Related allowance | 10 |  | 4
Recorded investment | 93 |  | 93
Unpaid principal balance | 122 |  | 122
Acquired Loans [Member] | Commercial Real Estate With Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 118 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 |
Related allowance | 2 |  | 29
Recorded investment | 109 |  | 137
Unpaid principal balance | 124 |  | 150
Acquired Loans [Member] | Loans To Individuals With Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 |
Related allowance | 0 |  | 0
Recorded investment | 0 |  | 0
Unpaid principal balance | 0 |  | 0
Acquired Loans [Member] | With Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 211 | 65 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 |
Related allowance | 12 |  | 33
Recorded investment | 202 |  | 230
Unpaid principal balance | 246 |  | 272
Originated Loans [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 44,781 | 43,975 |
Impaired Financing Receivable, Interest Income, Accrual Method | 173 | 202 |
Related allowance | 11,461 |  | 3,713
Recorded investment | 55,059 |  | 39,951
Unpaid principal balance | 62,380 |  | 53,021
Originated Loans [Member] | Commercial Loan [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 17,720 | 12,787 |
Impaired Financing Receivable, Interest Income, Accrual Method | 66 | 26 |
Related allowance | 9,045 |  | 3,478
Recorded investment | 28,615 |  | 16,866
Unpaid principal balance | 28,795 |  | 21,094
Originated Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 8,130 | 12,034 |
Impaired Financing Receivable, Interest Income, Accrual Method | 10 | 38 |
Recorded investment | 4,791 |  | 5,548
Unpaid principal balance | 9,559 |  | 12,153
Originated Loans [Member] | Real Estate Construction With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 |
Recorded investment | 0 |  | 0
Unpaid principal balance | 0 |  | 0
Originated Loans [Member] | Residential Real Estate With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 10,401 | 11,422 |
Impaired Financing Receivable, Interest Income, Accrual Method | 63 | 74 |
Recorded investment | 10,129 |  | 10,625
Unpaid principal balance | 12,000 |  | 12,470
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 5,510 | 5,949 |
Impaired Financing Receivable, Interest Income, Accrual Method | 31 | 54 |
Recorded investment | 4,624 |  | 5,155
Unpaid principal balance | 5,038 |  | 5,489
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 354 | 338 |
Impaired Financing Receivable, Interest Income, Accrual Method | 2 | 2 |
Recorded investment | 339 |  | 347
Unpaid principal balance | 379 |  | 383
Originated Loans [Member] | With No Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 24,395 | 29,743 |
Impaired Financing Receivable, Interest Income, Accrual Method | 106 | 168 |
Recorded investment | 19,883 |  | 21,675
Unpaid principal balance | 26,976 |  | 30,495
Originated Loans [Member] | Real Estate Construction With Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 |
Related allowance | 0 |  | 0
Recorded investment | 0 |  | 0
Unpaid principal balance | 0 |  | 0
Originated Loans [Member] | Residential Real Estate With Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 706 | 334 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 1 |
Related allowance | 277 |  | 107
Recorded investment | 808 |  | 456
Unpaid principal balance | 846 |  | 478
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 1,960 | 1,111 |
Impaired Financing Receivable, Interest Income, Accrual Method | 1 | 7 |
Related allowance | 2,139 |  | 128
Recorded investment | 5,753 |  | 954
Unpaid principal balance | 5,763 |  | 954
Originated Loans [Member] | Loans To Individuals With Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 |
Related allowance | 0 |  | 0
Recorded investment | 0 |  | 0
Unpaid principal balance | 0 |  | 0
Originated Loans [Member] | With Related Allowance [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Impaired Financing Receivable, Average Recorded Investment | 20,386 | 14,232 |
Impaired Financing Receivable, Interest Income, Accrual Method | 67 | $ 34 |
Related allowance | 11,461 |  | 3,713
Recorded investment | 35,176 |  | 18,276
Unpaid principal balance | $ 35,404 |  | $ 22,526

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