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Fair Value Measurements - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Fair Value Measurements, Inter-transfers between Levels $ 0 $ 0  
Decrease in fair value of contingent purchase price liabilities 2,128,000 1,342,000 $ 2,853,000
Other Income, net [Member]      
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Decrease in fair value of contingent purchase price liabilities $ 1,500,000 $ 1,300,000