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Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 526,143 $ 212,642
Gross Unrealized Gains 1 2
Gross Unrealized Losses (186) (31)
Fair Value 525,958 212,613
U.S. Government Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 89,592 49,612
Gross Unrealized Losses (61) (6)
Fair Value 89,531 49,606
U.S. Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 60,761 48,982
Gross Unrealized Gains 1 2
Gross Unrealized Losses (100) (25)
Fair Value 60,662 48,959
U.S. Commercial Paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 151,223 65,583
Gross Unrealized Losses (14)  
Fair Value 151,209 65,583
International Commercial Paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 224,567 48,465
Gross Unrealized Losses (11)  
Fair Value $ 224,556 $ 48,465