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Employee Benefit Plans - Summary of Amounts Recognized in Accumulated Other Comprehensive Income (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Pension Plan [Member]    
Amounts Recognized in Accumulated Other Comprehensive Income    
Net actuarial loss (gain) $ (992,724) $ 1,767,058
Total (before tax effects) (992,724) 1,767,058
Supplemental Employee Retirement Plan [Member]    
Amounts Recognized in Accumulated Other Comprehensive Income    
Net actuarial loss (gain) 421,264 (415,416)
Total (before tax effects) $ 421,264 (415,416)
Postretirement Benefits [Member]    
Amounts Recognized in Accumulated Other Comprehensive Income    
Net actuarial loss (gain)   (103,396)
Total (before tax effects)   $ (103,396)