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Commitments and Contingencies - Additional Information (Detail) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Long Term Purchase Commitment [Line Items]    
Unconditional purchase obligations, term 4 years  
Unconditional purchase obligations $ 72,835  
Bank of America, N.A.    
Long Term Purchase Commitment [Line Items]    
Amount outstanding under letters of credit $ 1,231 $ 854
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