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Employee Benefit Plans - Summary of Change in Plan Assets (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Pension Plan [Member]    
Change in Plan Assets    
Fair value of plan assets at beginning of year $ 83,868,508  
Actual return on plan assets 4,531,725 $ (1,731,372)
Employer contribution 0 0
Business combinations   86,008,964
Settlements (33,447,301)  
Benefits paid (2,749,560) (409,084)
Fair value of plan assets at end of year 52,203,372 83,868,508
Funded Status 5,852 (3,993,298)
Unrecognized net actuarial (gain) loss (992,724) 1,767,058
Cumulative employer contributions in excess of the net periodic pension cost (986,872) (2,226,240)
Supplemental Employee Retirement Plan [Member]    
Change in Plan Assets    
Employer contribution 312,088 38,550
Benefits paid (312,088) (38,550)
Funded Status (9,616,702) (8,751,848)
Unrecognized net actuarial (gain) loss 421,264 (415,416)
Cumulative employer contributions in excess of the net periodic pension cost (9,195,438) (9,167,264)
Postretirement Benefits [Member]    
Change in Plan Assets    
Employer contribution 47,860 18,457
Plan participants' contributions   1,939
Benefits paid $ (47,860) (20,396)
Funded Status   (1,756,423)
Unrecognized net actuarial (gain) loss   (103,396)
Cumulative employer contributions in excess of the net periodic pension cost   $ (1,859,819)