Fair Value Measurements and Available for Sale Investments - Assets Measured at Fair Value on Recurring Basis (Details) - Recurring Basis - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 40,965	$ 17,295
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	199,874	123,439
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	26,687
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	83,813
Quoted Market Prices for Identical Assets (Level 1) | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	40,965	17,295
Quoted Market Prices for Identical Assets (Level 1) | U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	199,874	$ 123,439
Significant Other Observable Inputs (Level 2) | Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	26,687
Significant Other Observable Inputs (Level 2) | Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 83,813