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Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 USD ($) Contract | Dec. 31, 2016 USD ($) Contract | Dec. 31, 2015 USD ($) Contract
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts | Contract | 41 | 65 | 61
Extend Maturity | $ 7,081 | $ 1,521 | $ 1,754
Modify Rate | 2,095 | 6,614 | 3,658
Modify Payments | 1,878 | 6,523 | 6,440
Total Pre-Modification Outstanding Recorded Investment | 11,054 | 14,658 | 11,852
Post-Modification Outstanding Recorded Investment | 8,274 | 10,275 | 10,956
Specific Reserve | $ 596 | $ 500 | $ 1,332
Commercial Financial Agricultural And Other [Member] |  |  |
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts | Contract | 6 | 5 | 12
Extend Maturity | $ 6,768 | $ 23 | $ 1,751
Modify Rate | 1,806 | 6,318 | 3,195
Modify Payments | 987 | 3,854 | 4,527
Total Pre-Modification Outstanding Recorded Investment | 9,561 | 10,195 | 9,473
Post-Modification Outstanding Recorded Investment | 6,946 | 6,210 | 8,823
Specific Reserve | $ 566 | $ 317 | $ 1,330
Residential real estate [Member] |  |  |
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts | Contract | 20 | 39 | 32
Extend Maturity | $ 134 | $ 107 | $ 0
Modify Rate | 261 | 214 | 296
Modify Payments | 573 | 2,619 | 1,414
Total Pre-Modification Outstanding Recorded Investment | 968 | 2,940 | 1,710
Post-Modification Outstanding Recorded Investment | 851 | 2,698 | 1,575
Specific Reserve | $ 1 | $ 124 | $ 2
Commercial real estate [Member] |  |  |
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts | Contract | 5 | 8 | 1
Extend Maturity | $ 179 | $ 1,368 | $ 0
Modify Rate | 0 | 0 | 0
Modify Payments | 269 | 25 | 464
Total Pre-Modification Outstanding Recorded Investment | 448 | 1,393 | 464
Post-Modification Outstanding Recorded Investment | 412 | 1,271 | 389
Specific Reserve | $ 29 | $ 59 | $ 0
Loans To Individuals [Member] |  |  |
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts | Contract | 10 | 13 | 16
Extend Maturity | $ 0 | $ 23 | $ 3
Modify Rate | 28 | 82 | 167
Modify Payments | 49 | 25 | 35
Total Pre-Modification Outstanding Recorded Investment | 77 | 130 | 205
Post-Modification Outstanding Recorded Investment | 65 | 96 | 169
Specific Reserve | $ 0 | $ 0 | $ 0

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