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Investments - Summary of Gross Unrealized Losses and Fair Value of Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value $ 96,610 $ 263,414
Less Than 12 Months, Unrealized Losses (201) (408)
12 Months or Greater, Estimated Fair Value 31,247  
12 Months or Greater, Unrealized Losses (129)  
Total, Estimated Fair Value 127,857 263,414
Total, Unrealized Losses (330) (408)
U.S. Treasury notes    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value   32,964
Less Than 12 Months, Unrealized Losses   (12)
Total, Estimated Fair Value   32,964
Total, Unrealized Losses   (12)
Government sponsored enterprise securities    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value   10,072
Less Than 12 Months, Unrealized Losses   (10)
Total, Estimated Fair Value   10,072
Total, Unrealized Losses   (10)
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 81,064 129,820
Less Than 12 Months, Unrealized Losses (189) (321)
12 Months or Greater, Estimated Fair Value 28,249  
12 Months or Greater, Unrealized Losses (128)  
Total, Estimated Fair Value 109,313 129,820
Total, Unrealized Losses (317) (321)
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 15,546 90,558
Less Than 12 Months, Unrealized Losses (12) (65)
12 Months or Greater, Estimated Fair Value 2,998  
12 Months or Greater, Unrealized Losses (1)  
Total, Estimated Fair Value 18,544 90,558
Total, Unrealized Losses $ (13) $ (65)