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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, 2016 USD ($) Contract | Dec. 31, 2015 USD ($) Contract | Dec. 31, 2014 USD ($) Contract
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts | Contract | 65 | 61 | 77
Extend Maturity | $ 1,521 | $ 1,754 | $ 5,487
Modify Rate | 6,614 | 3,658 | 732
Modify Payments | 6,523 | 6,440 | 16,381
Total Pre-Modification Outstanding Recorded Investment | 14,658 | 11,852 | 22,600
Post-Modification Outstanding Recorded Investment | 10,275 | 10,956 | 15,967
Specific Reserve | $ 500 | $ 1,332 | $ 4,665
Commercial Financial Agricultural And Other [Member] |  |  |
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts | Contract | 5 | 12 | 9
Extend Maturity | $ 23 | $ 1,751 | $ 5,487
Modify Rate | 6,318 | 3,195 | 0
Modify Payments | 3,854 | 4,527 | 14,529
Total Pre-Modification Outstanding Recorded Investment | 10,195 | 9,473 | 20,016
Post-Modification Outstanding Recorded Investment | 6,210 | 8,823 | 13,785
Specific Reserve | $ 317 | $ 1,330 | $ 4,665
Residential real estate [Member] |  |  |
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts | Contract | 39 | 32 | 52
Extend Maturity | $ 107 | $ 0 | $ 0
Modify Rate | 214 | 296 | 629
Modify Payments | 2,619 | 1,414 | 1,797
Total Pre-Modification Outstanding Recorded Investment | 2,940 | 1,710 | 2,426
Post-Modification Outstanding Recorded Investment | 2,698 | 1,575 | 2,062
Specific Reserve | $ 124 | $ 2 | $ 0
Commercial real estate [Member] |  |  |
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts | Contract | 8 | 1 | 1
Extend Maturity | $ 1,368 | $ 0 | $ 0
Modify Rate | 0 | 0 | 0
Modify Payments | 25 | 464 | 8
Total Pre-Modification Outstanding Recorded Investment | 1,393 | 464 | 8
Post-Modification Outstanding Recorded Investment | 1,271 | 389 | 6
Specific Reserve | $ 59 | $ 0 | $ 0
Loans To Individuals [Member] |  |  |
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts | Contract | 13 | 16 | 15
Extend Maturity | $ 23 | $ 3 | $ 0
Modify Rate | 82 | 167 | 103
Modify Payments | 25 | 35 | 47
Total Pre-Modification Outstanding Recorded Investment | 130 | 205 | 150
Post-Modification Outstanding Recorded Investment | 96 | 169 | 114
Specific Reserve | $ 0 | $ 0 | $ 0

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