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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 [Line Items]      
Capital lease assets $ 63,400 $ 64,200  
Assets fully depreciated 26,800 22,800  
Capital leased assets, net book value 13,000 16,400  
Rent expense under operating leases $ 15,123 $ 11,580 $ 9,362
Minimum [Member]      
Commitments And Contingencies Disclosure [Line Items]      
Estimated life of capital lease 4 years    
Noncancellable operating leases, renewal period 1 year    
Maximum [Member]      
Commitments And Contingencies Disclosure [Line Items]      
Estimated life of capital lease 6 years    
Noncancellable operating leases, renewal period 5 years