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Leases - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Operating Leased Assets [Line Items]      
Operating lease expiration year 2025    
Rent expense $ 6,900 $ 6,700 $ 7,200
Related Party Transaction      
Operating Leased Assets [Line Items]      
Rent expense $ 150 $ 330 $ 1,800