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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Liabilities:      
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net $ 0 $ 0 $ 0
Other Than Temporary Impairment Losses Investments Portion Recognized In Earnings Net, non current 0 $ 0 $ 0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]      
Assets:      
Money market instruments 8,631,000    
Corporate bonds 0    
Treasury bills 0    
Asset-backed securities 0    
Agency bonds 0    
Municipal bonds 0    
Economic hedge forward contracts 0    
Liabilities:      
Economic hedge forward contracts 0    
Significant Other Observable Inputs (Level 2) [Member]      
Assets:      
Money market instruments 0    
Corporate bonds 343,409,000    
Treasury bills 173,830,000    
Asset-backed securities 130,930,000    
Sovereign bonds 34,726,000    
Agency bonds 25,498,000    
Municipal bonds 13,009,000    
Economic hedge forward contracts 16,000    
Liabilities:      
Economic hedge forward contracts 13,000    
Fair Value, Inputs, Level 3 [Member]      
Liabilities:      
Contingent consideration liabilities $ 3,557,000