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Employee Benefit Plans - Summary of Amounts Recognized in Statement of Financial Position (Detail) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Pension Plan [Member]    
Amounts Recognized in the Statement of Financial Position    
Noncurrent assets $ 5,852  
Noncurrent liabilities   $ (3,993,298)
Net amount recognized 5,852 (3,993,298)
Supplemental Employee Retirement Plan [Member]    
Amounts Recognized in the Statement of Financial Position    
Current liabilities (505,169) (422,656)
Noncurrent liabilities (9,111,533) (8,329,192)
Net amount recognized $ (9,616,702) (8,751,848)
Postretirement Benefits [Member]    
Amounts Recognized in the Statement of Financial Position    
Current liabilities   (113,268)
Noncurrent liabilities   (1,643,155)
Net amount recognized   $ (1,756,423)