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Employee Benefits - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Benefits expected to be paid in 2020 $ 1,798,000    
Benefits expected to be paid in 2021 2,023,000    
Benefits expected to be paid in 2022 2,157,000    
Benefits expected to be paid in 2023 2,270,000    
Benefits expected to be paid in 2024 2,451,000    
Aggregate benefits expected to paid $ 15,005,000    
Voluntary contribution of employees 33.30%    
Contributions matched by compensation contribution 6.00%    
Voluntary contribution of employees, amount $ 458,000 $ 454,000 $ 445,000
Discretionary bonus expense 2,364,000 2,355,000 $ 1,859,000
Change in Assumptions for Pension Plans [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Decrease in the projected benefit obligations   $ 6,800,000  
Supplemental Insurance/ Retirement Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Benefits expected to be paid in 2020 2,373,000    
Benefits expected to be paid in 2021 2,318,000    
Benefits expected to be paid in 2022 2,409,000    
Benefits expected to be paid in 2023 2,692,000    
Benefits expected to be paid in 2024 3,138,000    
Aggregate benefits expected to paid $ 18,017,000    
Equity Securities [Member] | Minimum [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Allocation mix for the common and collective trust portfolio 43.00%    
Equity Securities [Member] | Maximum [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Allocation mix for the common and collective trust portfolio 57.00%    
Fixed Income Funds [Member] | Minimum [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Allocation mix for the common and collective trust portfolio 15.00%    
Fixed Income Funds [Member] | Maximum [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Allocation mix for the common and collective trust portfolio 25.00%    
Hedge Funds [Member] | Minimum [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Allocation mix for the common and collective trust portfolio 15.00%    
Hedge Funds [Member] | Maximum [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Allocation mix for the common and collective trust portfolio 31.00%