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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 | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Acquired Loans [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | $ 4,585 | $ 4,844 | $ 3,614 | $ 3,189 |
Impaired Financing Receivable, Interest Income, Accrual Method | 12 | 1 | 13 | 1 |
Related allowance | 0 |  | 0 |  | $ 33
Recorded investment | 4,444 |  | 4,444 |  | 2,289
Unpaid principal balance | 6,015 |  | 6,015 |  | 3,545
Acquired Loans [Member] | Commercial Loan [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 786 | 0 | 316 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 0
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 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 | 73 | 72 | 261 | 48 |
Impaired Financing Receivable, Interest Income, Accrual Method | 10 | 1 | 10 | 1 |
Recorded investment | 73 |  | 73 |  | 436
Unpaid principal balance | 73 |  | 73 |  | 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 | 0 | 33 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 0 |  | 0
Acquired Loans [Member] | Residential Real Estate With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 2,541 | 653 | 1,779 | 487 |
Impaired Financing Receivable, Interest Income, Accrual Method | 2 | 0 | 3 | 0 |
Recorded investment | 2,436 |  | 2,436 |  | 666
Unpaid principal balance | 3,006 |  | 3,006 |  | 965
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 1,955 | 3,078 | 1,558 | 2,076 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Recorded investment | 1,920 |  | 1,920 |  | 940
Unpaid principal balance | 2,919 |  | 2,919 |  | 1,842
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 16 | 6 | 16 | 2 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Recorded investment | 15 |  | 15 |  | 17
Unpaid principal balance | 17 |  | 17 |  | 17
Acquired Loans [Member] | With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 4,585 | 3,809 | 3,614 | 2,646 |
Impaired Financing Receivable, Interest Income, Accrual Method | 12 | 1 | 13 | 1 |
Recorded investment | 4,444 |  | 4,444 |  | 2,059
Unpaid principal balance | 6,015 |  | 6,015 |  | 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 | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 0
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 0 |  | 0
Acquired Loans [Member] | Residential Real Estate With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 95 | 0 | 68 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 4
Recorded investment | 0 |  | 0 |  | 93
Unpaid principal balance | 0 |  | 0 |  | 122
Acquired Loans [Member] | Commercial Real Estate With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 154 | 0 | 159 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 29
Recorded investment | 0 |  | 0 |  | 137
Unpaid principal balance | 0 |  | 0 |  | 150
Acquired Loans [Member] | Loans To Individuals With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 0
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 0 |  | 0
Acquired Loans [Member] | With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 1,035 | 0 | 543 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 33
Recorded investment | 0 |  | 0 |  | 230
Unpaid principal balance | 0 |  | 0 |  | 272
Originated Loans [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 38,659 | 33,819 | 44,729 | 39,437 |
Impaired Financing Receivable, Interest Income, Accrual Method | 135 | 637 | 949 | 1,016 |
Related allowance | 5,363 |  | 5,363 |  | 3,713
Recorded investment | 35,405 |  | 35,405 |  | 39,951
Unpaid principal balance | 46,703 |  | 46,703 |  | 53,021
Originated Loans [Member] | Commercial Loan [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 10,298 | 8,510 | 5,979 | 8,984 |
Impaired Financing Receivable, Interest Income, Accrual Method | 4 | 32 | 16 | 77 |
Related allowance | 3,474 |  | 3,474 |  | 3,478
Recorded investment | 9,998 |  | 9,998 |  | 16,866
Unpaid principal balance | 10,219 |  | 10,219 |  | 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 | 5,697 | 7,106 | 17,838 | 11,627 |
Impaired Financing Receivable, Interest Income, Accrual Method | 8 | 62 | 576 | 142 |
Recorded investment | 3,177 |  | 3,177 |  | 5,548
Unpaid principal balance | 9,585 |  | 9,585 |  | 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 | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 0 |  | 0
Originated Loans [Member] | Residential Real Estate With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 10,627 | 11,217 | 10,639 | 11,417 |
Impaired Financing Receivable, Interest Income, Accrual Method | 59 | 82 | 191 | 245 |
Recorded investment | 10,302 |  | 10,302 |  | 10,625
Unpaid principal balance | 12,163 |  | 12,163 |  | 12,470
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 6,810 | 5,928 | 7,632 | 6,439 |
Impaired Financing Receivable, Interest Income, Accrual Method | 59 | 452 | 146 | 522 |
Recorded investment | 4,951 |  | 4,951 |  | 5,155
Unpaid principal balance | 7,559 |  | 7,559 |  | 5,489
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 320 | 366 | 322 | 350 |
Impaired Financing Receivable, Interest Income, Accrual Method | 2 | 5 | 6 | 16 |
Recorded investment | 287 |  | 287 |  | 347
Unpaid principal balance | 414 |  | 414 |  | 383
Originated Loans [Member] | With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 23,454 | 24,617 | 36,431 | 29,833 |
Impaired Financing Receivable, Interest Income, Accrual Method | 128 | 601 | 919 | 925 |
Recorded investment | 18,717 |  | 18,717 |  | 21,675
Unpaid principal balance | 29,721 |  | 29,721 |  | 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 | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 0
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 0 |  | 0
Originated Loans [Member] | Residential Real Estate With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 565 | 377 | 532 | 266 |
Impaired Financing Receivable, Interest Income, Accrual Method | 2 | 0 | 11 | 0 |
Related allowance | 167 |  | 167 |  | 107
Recorded investment | 596 |  | 596 |  | 456
Unpaid principal balance | 602 |  | 602 |  | 478
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 4,342 | 315 | 1,787 | 354 |
Impaired Financing Receivable, Interest Income, Accrual Method | 1 | 4 | 3 | 14 |
Related allowance | 1,722 |  | 1,722 |  | 128
Recorded investment | 6,094 |  | 6,094 |  | 954
Unpaid principal balance | 6,161 |  | 6,161 |  | 954
Originated Loans [Member] | Loans To Individuals With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 | 0 | 0 | 0 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 0 |  | 0 |  | 0
Recorded investment | 0 |  | 0 |  | 0
Unpaid principal balance | 0 |  | 0 |  | 0
Originated Loans [Member] | With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 15,205 | 9,202 | 8,298 | 9,604 |
Impaired Financing Receivable, Interest Income, Accrual Method | 7 | $ 36 | 30 | $ 91 |
Related allowance | 5,363 |  | 5,363 |  | 3,713
Recorded investment | 16,688 |  | 16,688 |  | 18,276
Unpaid principal balance | $ 16,982 |  | $ 16,982 |  | $ 22,526

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