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Long-Term Debt - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Debt Instrument [Line Items]        
Principal payments on long -term debt due in 2022 $ 97,400,000      
Secured Revolving Credit Facility        
Debt Instrument [Line Items]        
Average line of credit facility outstanding $ 93,346,000 $ 51,103,000    
Weighted average interest rate 3.60% 3.00%   2.30%
Maximum        
Debt Instrument [Line Items]        
Available borrowings capacity under credit agreement     $ 250,000,000  
Revolving Credit Facility        
Debt Instrument [Line Items]        
Available borrowings capacity under credit agreement   $ 139,241,000    
Revolving Credit Facility | Maximum        
Debt Instrument [Line Items]        
Capacity to increase borrowings under credit agreement $ 112,150,000