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Income Taxes (Details) - Tax Rate Reconciliation (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Sep. 30, 2016 | Sep. 30, 2015 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Tax Rate Reconciliation [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Provision at statutory rate |  |  |  |  |  |  |  |  |  |  |  | $ 2,866 | $ 3,119 | $ 3,062
Tax-free income |  |  |  |  |  |  |  |  |  |  |  | (1,347) | (1,187) | (1,138)
Nondeductible interest expense |  |  |  |  |  |  |  |  |  |  |  | 34 | 37 | 45
Other |  |  |  |  |  |  |  |  |  |  |  | 9 | 23 | 10
Actual tax expense | $ 261 | $ 298 | $ 544 | $ 316 | $ 404 | $ 550 | $ 529 | $ 414 | $ 499 | $ 1,129 | $ 1,264 | $ 1,562 | $ 1,992 | $ 1,979
Provision at statutory rate |  |  |  |  |  |  |  |  |  |  |  | 34.00% | 34.00% | 34.00%
Tax-free income |  |  |  |  |  |  |  |  |  |  |  | (15.90%) | (12.90%) | (12.60%)
Nondeductible interest expense |  |  |  |  |  |  |  |  |  |  |  | 0.40% | 0.40% | 0.50%
Other |  |  |  |  |  |  |  |  |  |  |  | 0.00% | 0.20% | 0.10%
Effective rate |  |  |  |  |  |  |  |  |  |  |  | 18.50% | 21.70% | 22.00%

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