Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	$ 85,258	$ 118,902
Level 1
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	23,405	22,275
Level 2
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	61,853	96,627
U.S. Government and Agency Obligations
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	41,573	42,264
U.S. Government and Agency Obligations | Level 1
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	23,405	22,275
U.S. Government and Agency Obligations | Level 2
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	18,168	19,989
Certificates of Deposit
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	5,961	5,408
Certificates of Deposit | Level 2
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	5,961	5,408
Commercial Paper
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	24,603	47,686
Commercial Paper | Level 2
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	24,603	47,686
Corporate Debt Securities
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	13,121	23,544
Corporate Debt Securities | Level 2
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Total assets measured at fair value	$ 13,121	$ 23,544