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Commitments and Contingencies - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Other Commitments [Line Items]    
Outstanding letter of credit, amount $ 1,100,000 $ 1,100,000
Amounts drawn against outstanding letters of credit or performance bond 0 0
Restricted Escrow Deposits 1,000,000 800,000
Performance Bonds [Member]    
Other Commitments [Line Items]    
Outstanding performance and payment of bonds $ 4,000,000 $ 4,200,000