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Indebtedness - Additional Information (Detail) - USD ($)
6 Months Ended
Jun. 30, 2017
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Debt Instrument [Line Items]        
Short term debt   $ 0 $ 0  
Senior secured credit facility, maturity date Jun. 30, 2022      
Revolving Line of Credit        
Debt Instrument [Line Items]        
Available borrowings capacity under credit agreement       $ 95,985,000
Maximum        
Debt Instrument [Line Items]        
Available borrowings capacity under credit agreement $ 250,000,000      
Consolidated funded debt ratio 325.00%      
Maximum | Revolving Line of Credit        
Debt Instrument [Line Items]        
Capacity to increase borrowings under credit agreement   $ 124,724,000    
Maximum | Term Loan        
Debt Instrument [Line Items]        
Accordion feature $ 100,000,000      
Minimum        
Debt Instrument [Line Items]        
Consolidated fixed charge covenant ratio 125.00%