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Investments - Summary of Cash, Cash Equivalents and Investments (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Dec. 31, 2015
Cash and cash equivalents:        
AMORTIZED COST $ 194,078 $ 197,945 $ 211,938 $ 91,060
FAIR VALUE 194,078 197,945    
Investments:        
AMORTIZED COST 282,333 233,730    
GROSS UNREALIZED GAINS 0 8    
GROSS UNREALIZED LOSSES (98) (76)    
FAIR VALUE 282,235 233,662    
Cash and money market funds        
Cash and cash equivalents:        
AMORTIZED COST 194,078 196,445    
FAIR VALUE 194,078 196,445    
Commercial paper        
Cash and cash equivalents:        
AMORTIZED COST   1,500    
FAIR VALUE   1,500    
Commercial paper        
Investments:        
AMORTIZED COST 43,225      
GROSS UNREALIZED GAINS 0      
GROSS UNREALIZED LOSSES 0      
FAIR VALUE 43,225      
Total investments        
Investments:        
AMORTIZED COST 88,255 35,785    
GROSS UNREALIZED GAINS 0 8    
GROSS UNREALIZED LOSSES (98) (76)    
FAIR VALUE 88,157 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 44,125 27,615    
GROSS UNREALIZED GAINS 0 4    
GROSS UNREALIZED LOSSES (94) (75)    
FAIR VALUE 44,031 27,544    
Corporate bonds (due within 2 years)        
Investments:        
AMORTIZED COST 905      
GROSS UNREALIZED GAINS 0      
GROSS UNREALIZED LOSSES (4)      
FAIR VALUE $ 901      
Government agencies        
Investments:        
AMORTIZED COST   1,250    
GROSS UNREALIZED GAINS   0    
GROSS UNREALIZED LOSSES   0    
FAIR VALUE   $ 1,250