Fair Value Measurements - Schedule of Fair Value, Assets Measured on Recurring Basis (Details) - Fair Value On Recurring Basis - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	$ 265,583	$ 175,703
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	58,404	66,478
U.S. Government Agency Obligations
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	89,461	57,153
Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	117,718	52,072
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	58,404	66,478
Level 1 | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	58,404	66,478
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	207,179	109,225
Level 2 | U.S. Government Agency Obligations
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	89,461	57,153
Level 2 | Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	$ 117,718	$ 52,072