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Line of Credit - Additional Information (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Oct. 26, 2018
Dec. 31, 2017
Jun. 30, 2015
Line of Credit Facility [Line Items]          
Proceeds from Lines of Credit $ 20,000 $ 40,000      
Long-term debt, gross 0     $ 0  
Repayments of Lines of Credit $ 20,000 $ 40,000      
Senior Unsecured Revolving Line Of Credit Member [Member]          
Line of Credit Facility [Line Items]          
Senior unsecured revolving line of credit aggregate commitment         $ 100,000
Sublimit for letters of credit         25,000
Line of credit facility expansion feature         $ 50,000
Subsequent Event [Member]          
Line of Credit Facility [Line Items]          
Senior unsecured revolving line of credit aggregate commitment     $ 175,000    
Sublimit for letters of credit     25,000    
Line of credit facility expansion feature     $ 75,000