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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, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016
Acquired Loans [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | $ 4,844 |  | $ 3,189 |  |
Impaired Financing Receivable, Interest Income, Accrual Method | 1 |  | 1 |  |
Related allowance | 521 |  | 521 |  | $ 16
Recorded investment | 5,024 |  | 5,024 |  | 587
Unpaid principal balance | 6,287 |  | 6,287 |  | 661
Acquired Loans [Member] | Commercial Loan [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 786 |  | 316 |  |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 |  | 0 |  |
Related allowance | 472 |  | 472 |  | 0
Recorded investment | 934 |  | 934 |  | 0
Unpaid principal balance | 940 |  | 940 |  | 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 | 72 |  | 48 |  |
Impaired Financing Receivable, Interest Income, Accrual Method | 1 |  | 1 |  |
Recorded investment | 72 |  | 72 |  | 0
Unpaid principal balance | 72 |  | 72 |  | 0
Acquired Loans [Member] | Real Estate Construction With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 0 |  | 33 |  |
Impaired Financing Receivable, Interest Income, Accrual Method | 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 | 653 |  | 487 |  |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 |  | 0 |  |
Recorded investment | 707 |  | 707 |  | 406
Unpaid principal balance | 1,015 |  | 1,015 |  | 480
Acquired Loans [Member] | Commercial Real Estate With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 3,078 |  | 2,076 |  |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 |  | 0 |  |
Recorded investment | 3,047 |  | 3,047 |  | 162
Unpaid principal balance | 3,961 |  | 3,961 |  | 162
Acquired Loans [Member] | Loans To Individuals With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 6 |  | 2 |  |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 |  | 0 |  |
Recorded investment | 17 |  | 17 |  | 3
Unpaid principal balance | 17 |  | 17 |  | 3
Acquired Loans [Member] | With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 3,809 |  | 2,646 |  |
Impaired Financing Receivable, Interest Income, Accrual Method | 1 |  | 1 |  |
Recorded investment | 3,843 |  | 3,843 |  | 571
Unpaid principal balance | 5,065 |  | 5,065 |  | 645
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 |  | 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 | 95 |  | 68 |  |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 |  | 0 |  |
Related allowance | 5 |  | 5 |  | 16
Recorded investment | 94 |  | 94 |  | 16
Unpaid principal balance | 123 |  | 123 |  | 16
Acquired Loans [Member] | Commercial Real Estate With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 154 |  | 159 |  |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 |  | 0 |  |
Related allowance | 44 |  | 44 |  | 0
Recorded investment | 153 |  | 153 |  | 0
Unpaid principal balance | 159 |  | 159 |  | 0
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 |  | 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 | 1,035 |  | 543 |  |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 |  | 0 |  |
Related allowance | 521 |  | 521 |  | 16
Recorded investment | 1,181 |  | 1,181 |  | 16
Unpaid principal balance | 1,222 |  | 1,222 |  | 16
Originated Loans [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 33,819 | $ 61,085 | 39,437 | $ 59,083 |
Impaired Financing Receivable, Interest Income, Accrual Method | 637 | 331 | 1,016 | 942 |
Related allowance | 1,660 |  | 1,660 |  | 3,128
Recorded investment | 33,778 |  | 33,778 |  | 41,226
Unpaid principal balance | 47,151 |  | 47,151 |  | 56,296
Originated Loans [Member] | Commercial Loan [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 8,510 | 17,207 | 8,984 | 15,310 |
Impaired Financing Receivable, Interest Income, Accrual Method | 32 | 22 | 77 | 77 |
Related allowance | 1,346 |  | 1,346 |  | 2,530
Recorded investment | 8,394 |  | 8,394 |  | 13,423
Unpaid principal balance | 12,631 |  | 12,631 |  | 19,226
Originated Loans [Member] | Commercial Financial Agricultural And Other With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 7,106 | 24,674 | 11,627 | 24,020 |
Impaired Financing Receivable, Interest Income, Accrual Method | 62 | 128 | 142 | 421 |
Recorded investment | 7,028 |  | 7,028 |  | 9,549
Unpaid principal balance | 13,464 |  | 13,464 |  | 15,369
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 | 6 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 44 |
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 | 11,217 | 11,636 | 11,417 | 11,546 |
Impaired Financing Receivable, Interest Income, Accrual Method | 82 | 94 | 245 | 232 |
Recorded investment | 10,970 |  | 10,970 |  | 10,873
Unpaid principal balance | 12,810 |  | 12,810 |  | 13,004
Originated Loans [Member] | Commercial Real Estate With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 5,928 | 6,463 | 6,439 | 7,143 |
Impaired Financing Receivable, Interest Income, Accrual Method | 452 | 73 | 522 | 140 |
Recorded investment | 6,203 |  | 6,203 |  | 5,765
Unpaid principal balance | 6,958 |  | 6,958 |  | 6,905
Originated Loans [Member] | Loans To Individuals With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 366 | 384 | 350 | 423 |
Impaired Financing Receivable, Interest Income, Accrual Method | 5 | 7 | 16 | 10 |
Recorded investment | 357 |  | 357 |  | 382
Unpaid principal balance | 417 |  | 417 |  | 507
Originated Loans [Member] | With No Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 24,617 | 43,157 | 29,833 | 43,138 |
Impaired Financing Receivable, Interest Income, Accrual Method | 601 | 302 | 925 | 847 |
Recorded investment | 24,558 |  | 24,558 |  | 26,569
Unpaid principal balance | 33,649 |  | 33,649 |  | 35,785
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 | 377 | 174 | 266 | 111 |
Impaired Financing Receivable, Interest Income, Accrual Method | 0 | 0 | 0 | 0 |
Related allowance | 97 |  | 97 |  | 164
Recorded investment | 517 |  | 517 |  | 424
Unpaid principal balance | 562 |  | 562 |  | 475
Originated Loans [Member] | Commercial Real Estate With Related Allowance [Member] |  |  |  |  |
Financing Receivable, Impaired [Line Items] |  |  |  |  |
Impaired Financing Receivable, Average Recorded Investment | 315 | 547 | 354 | 524 |
Impaired Financing Receivable, Interest Income, Accrual Method | 4 | 7 | 14 | 18 |
Related allowance | 217 |  | 217 |  | 434
Recorded investment | 309 |  | 309 |  | 810
Unpaid principal balance | 309 |  | 309 |  | 810
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 | 9,202 | 17,928 | 9,604 | 15,945 |
Impaired Financing Receivable, Interest Income, Accrual Method | 36 | $ 29 | 91 | $ 95 |
Related allowance | 1,660 |  | 1,660 |  | 3,128
Recorded investment | 9,220 |  | 9,220 |  | 14,657
Unpaid principal balance | $ 13,502 |  | $ 13,502 |  | $ 20,511

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