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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
Jun. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value $ 160,369 $ 263,414
Less Than 12 Months, Unrealized Losses (516) (408)
Total, Estimated Fair Value 160,369 263,414
Total, Unrealized Losses (516) (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 136,308 129,820
Less Than 12 Months, Unrealized Losses (502) (321)
Total, Estimated Fair Value 136,308 129,820
Total, Unrealized Losses (502) (321)
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 24,061 90,558
Less Than 12 Months, Unrealized Losses (14) (65)
Total, Estimated Fair Value 24,061 90,558
Total, Unrealized Losses $ (14) $ (65)