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Commitments and Contingencies - Additional Information (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Long Term Purchase Commitment [Line Items]    
Unconditional purchase obligations, term 4 years  
Unconditional purchase obligations $ 65,841  
Bank of America, N.A.    
Long Term Purchase Commitment [Line Items]    
Amount outstanding under letters of credit $ 1,205 $ 857
Minimum    
Long Term Purchase Commitment [Line Items]    
Unconditional purchase obligations, term 1 year  
Maximum    
Long Term Purchase Commitment [Line Items]    
Unconditional purchase obligations, term 4 years