Fair Value Measurements - Summary of Fair Value Measurement of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 427,685	$ 275,588
Unrealized Loss	(254)	(187)
Fair Value	427,939	275,401
Cash equivalents (due within 90 days)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	348,525	223,568
Fair Value	348,525	223,568
Short-term investments (due within one year)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	51,576	32,010
Unrealized Loss	(190)	(77)
Fair Value	51,766	31,933
Long-term investments (due between one and two years)
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	27,584	20,010
Unrealized Loss	(64)	(110)
Fair Value	$ 27,648	$ 19,900