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Investments - Summary of Cash, Cash Equivalents and Investments (Detail) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2015
Cash and cash equivalents:        
AMORTIZED COST $ 242,650 $ 197,945 $ 58,519 $ 91,060
FAIR VALUE 242,650 197,945    
Investments:        
AMORTIZED COST 308,001 233,730    
GROSS UNREALIZED GAINS 0 8    
GROSS UNREALIZED LOSSES (82) (76)    
FAIR VALUE 307,919 233,662    
Cash and money market accounts        
Cash and cash equivalents:        
AMORTIZED COST 242,650 196,445    
FAIR VALUE 242,650 196,445    
Commercial paper        
Cash and cash equivalents:        
AMORTIZED COST   1,500    
FAIR VALUE   1,500    
Commercial paper        
Investments:        
AMORTIZED COST 33,480      
GROSS UNREALIZED GAINS 0      
GROSS UNREALIZED LOSSES 0      
FAIR VALUE 33,480      
Total investments        
Investments:        
AMORTIZED COST 65,351 35,785    
GROSS UNREALIZED GAINS 0 8    
GROSS UNREALIZED LOSSES (82) (76)    
FAIR VALUE 65,269 35,717    
Certificates of deposit (due within 1 year)        
Investments:        
AMORTIZED COST   6,920    
GROSS UNREALIZED GAINS   4    
GROSS UNREALIZED LOSSES   (1)    
FAIR VALUE   6,923    
Corporate bonds (due within 1 year)        
Investments:        
AMORTIZED COST 31,871 27,615    
GROSS UNREALIZED GAINS 0 4    
GROSS UNREALIZED LOSSES (82) (75)    
FAIR VALUE $ 31,789 27,544    
Government agencies        
Investments:        
AMORTIZED COST   1,250    
GROSS UNREALIZED GAINS   0    
GROSS UNREALIZED LOSSES   0    
FAIR VALUE   $ 1,250