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Credit Facility - Additional Information (Detail) - USD ($)
1 Months Ended 12 Months Ended
Mar. 31, 2014
Dec. 31, 2017
Dec. 31, 2016
GECC      
Line of Credit Facility [Line Items]      
Line of credit facility maximum borrowing capacity $ 75,000,000 $ 14,100,000 $ 12,100,000
Line of credit facility, maturity date   Mar. 27, 2019  
Amount of credit facility outstanding   $ 5,000,000 10,000,000
Stand by letters of credit outstanding amount   $ 20,000,000 $ 28,200,000
Rate of credit facility   3.79% 2.45%
GECC | London Interbank Offered Rate (LIBOR)      
Line of Credit Facility [Line Items]      
Applicable margin spread over base rate 2.25%    
GECC | Base Rate      
Line of Credit Facility [Line Items]      
Applicable margin spread over base rate 1.25%    
GECC | Minimum      
Line of Credit Facility [Line Items]      
Percentage of unused credit line fee   0.25%  
GECC | Maximum      
Line of Credit Facility [Line Items]      
Percentage of unused credit line fee   0.50%  
WF Loan Agreement | Minimum      
Line of Credit Facility [Line Items]      
Fixed charge coverage ratio   125.00%