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Employee Benefit Plans - Narrative (Detail)
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
SERP_retirement_age
Dec. 31, 2017
USD ($)
year
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Retirement Benefits [Abstract]              
Employers matching contribution     50.00%        
Employees first contribution     6.00%        
401(k) profit sharing plan, employers matching contribution amount   $ 1,900,000       $ 1,600,000 $ 1,100,000
SERP annual retirement benefits period (years) 15 years            
SERP retirement age       65 65    
SERP target annual retirement benefit $ 300,000 300,000 $ 300,000 $ 300,000 $ 300,000    
SERP annual rate of return     7.00%        
SERP present value of the amount owed $ 4,600,000 $ 4,600,000 $ 4,600,000 $ 4,600,000 $ 4,600,000