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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 | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | $ 50,844 | $ 55,251 |
Unpaid Principal Balance | 61,835 | 60,915 |
Related Allowance | 7,045 | 9,461 |
Average Recorded Investment | 45,458 | 49,794 | $ 78,175
Interest Income Recognized | 615 | 625 | 679
Commercial, financial, agricultural and other with no related allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 11,344 | 9,439 |
Unpaid Principal Balance | 15,673 | 10,937 |
Average Recorded Investment | 17,692 | 11,536 | 14,454
Interest Income Recognized | 216 | 133 | 73
Real estate construction with no related allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 28 | 236 |
Unpaid Principal Balance | 117 | 476 |
Average Recorded Investment | 95 | 1,190 | 5,923
Interest Income Recognized | 0 | 19 | 47
Residential real estate with no related allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 9,952 | 10,773 |
Unpaid Principal Balance | 11,819 | 12,470 |
Average Recorded Investment | 10,635 | 11,592 | 9,280
Interest Income Recognized | 172 | 210 | 211
Commercial real estate with no related allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 7,562 | 8,768 |
Unpaid Principal Balance | 9,449 | 10,178 |
Average Recorded Investment | 7,890 | 8,830 | 27,881
Interest Income Recognized | 90 | 98 | 250
Loans to individuals with no related allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 421 | 288 |
Unpaid Principal Balance | 507 | 337 |
Average Recorded Investment | 338 | 308 | 255
Interest Income Recognized | 4 | 4 | 3
Subtotal with no related allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 29,307 | 29,504 |
Unpaid Principal Balance | 37,565 | 34,398 |
Average Recorded Investment | 36,650 | 33,456 | 57,793
Interest Income Recognized | 482 | 464 | 584
Commercial, financial, agricultural and other With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 20,132 | 24,826 |
Unpaid Principal Balance | 22,590 | 25,583 |
Related Allowance | 6,952 | 9,304 |
Average Recorded Investment | 7,731 | 15,797 | 16,479
Interest Income Recognized | 129 | 143 | 64
Real estate construction with an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 0 | 0 |
Unpaid Principal Balance | 0 | 0 |
Related Allowance | 0 | 0 |
Average Recorded Investment | 0 | 0 | 515
Interest Income Recognized | 0 | 0 | 0
Residential real estate With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 461 | 367 |
Unpaid Principal Balance | 672 | 380 |
Related Allowance | 51 | 56 |
Average Recorded Investment | 403 | 357 | 3,200
Interest Income Recognized | 0 | 14 | 31
Commercial real estate With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 944 | 554 |
Unpaid Principal Balance | 1,008 | 554 |
Related Allowance | 42 | 101 |
Average Recorded Investment | 674 | 184 | 188
Interest Income Recognized | 4 | 4 | 0
Loans to individuals With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 0 | 0 |
Unpaid Principal Balance | 0 | 0 |
Related Allowance | 0 | 0 |
Average Recorded Investment | 0 | 0 | 0
Interest Income Recognized | 0 | 0 | 0
Subtotal With an allowance recorded [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Investment | 21,537 | 25,747 |
Unpaid Principal Balance | 24,270 | 26,517 |
Related Allowance | 7,045 | 9,461 |
Average Recorded Investment | 8,808 | 16,338 | 20,382
Interest Income Recognized | $ 133 | $ 161 | $ 95

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