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Commitments and Contingencies - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]      
Lease expiration date 2026    
Rental expenses under operating leases $ 19,693 $ 16,253 $ 7,845
Purchase commitments covered by aggregate value $ 278,797    
Purchase commitments Less than one year