Fair Value of Financial Instruments - Assets Measured or Disclosed at Fair Value on Recurring Basis (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Fair Value	$ 62,741,000	$ 104,644,000
Assets Measured at fair value on recurring basis	105,297,000
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	0	0
Fair value measurements recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Fair Value	62,741,000	104,644,000
Assets Measured at fair value on recurring basis	168,038,000	321,640,000
Fair value measurements recurring | Certificates of deposit
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	244,000	245,000
Fair value measurements recurring | U.S. government debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	89,518,000	158,321,000
Fair value measurements recurring | Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	15,535,000	58,430,000
Fair value measurements recurring | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents, Fair Value	62,741,000	104,644,000
Assets Measured at fair value on recurring basis	62,741,000	104,644,000
Fair value measurements recurring | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	105,297,000	216,996,000
Fair value measurements recurring | Level 2 | Certificates of deposit
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	244,000	245,000
Fair value measurements recurring | Level 2 | U.S. government debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	89,518,000	158,321,000
Fair value measurements recurring | 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	$ 15,535,000	$ 58,430,000