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Employee Benefit Plans - Summary of Components of Net Periodic Benefit Cost and Other Amounts Recognized in Other Comprehensive Income (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Pension Plan [Member]    
Net periodic benefit cost    
Interest cost $ 2,958,903 $ 530,839
Expected return on plan assets (3,562,150) (1,000,732)
Recognized actuarial (gain) loss due to settlements (636,121)  
Net periodic benefit cost (1,239,368) (469,893)
Supplemental Employee Retirement Plan [Member]    
Net periodic benefit cost    
Interest cost 340,262 51,696
Net periodic benefit cost 340,262 51,696
Postretirement Benefits [Member]    
Net periodic benefit cost    
Service cost 11,860 8,169
Interest cost 26,269 11,097
Recognized actuarial (gain) loss (2,079)  
Curtailment charge (credit) (1,678,920)  
Recognized actuarial (gain) loss due to settlements (169,089)  
Net periodic benefit cost $ (1,811,959) $ 19,266