Fair Value of Financial Instruments - Schedule of Fair Value Measurement (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Total investments	$ 100,218	$ 80,648
Recurring
Investment [Line Items]
Total cash and cash equivalents	39,937
Total investments	100,218	80,648
Total assets measured at fair value	140,155	88,480
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total cash and cash equivalents	38,444
Total investments	22,120	14,716
Total assets measured at fair value	60,564	22,548
Recurring | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total cash and cash equivalents	1,493
Total investments	78,098	65,932
Total assets measured at fair value	79,591	65,932
Recurring | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total cash and cash equivalents	0
Total investments	0	0
Total assets measured at fair value	0	0
Recurring | Certificates of deposit
Investment [Line Items]
Total investments	10,787	13,441
Recurring | Certificates of deposit | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	0	0
Recurring | Certificates of deposit | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	10,787	13,441
Recurring | Certificates of deposit | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
Recurring | US Treasury securities
Investment [Line Items]
Total investments	22,120	14,716
Recurring | US Treasury securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	22,120	14,716
Recurring | US Treasury securities | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	0	0
Recurring | US Treasury securities | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
Recurring | US Agency securities
Investment [Line Items]
Total investments	7,980	8,459
Recurring | US Agency securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	0	0
Recurring | US Agency securities | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	7,980	8,459
Recurring | US Agency securities | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
Recurring | Commercial paper
Investment [Line Items]
Total investments	17,025	9,171
Recurring | Commercial paper | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	0	0
Recurring | Commercial paper | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	17,025	9,171
Recurring | Commercial paper | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
Recurring | Asset-backed securities
Investment [Line Items]
Total investments	11,998	3,025
Recurring | Asset-backed securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	0	0
Recurring | Asset-backed securities | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	11,998	3,025
Recurring | Asset-backed securities | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
Recurring | Corporate notes and bonds
Investment [Line Items]
Total investments	30,308	31,836
Recurring | Corporate notes and bonds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	0	0
Recurring | Corporate notes and bonds | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	30,308	31,836
Recurring | Corporate notes and bonds | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
Recurring | Money market funds
Investment [Line Items]
Total cash and cash equivalents	38,444	7,832
Recurring | Money market funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total cash and cash equivalents	38,444	7,832
Recurring | Money market funds | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total cash and cash equivalents	0	0
Recurring | Money market funds | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total cash and cash equivalents	0	$ 0
Recurring | Commercial paper
Investment [Line Items]
Total cash and cash equivalents	1,493
Recurring | Commercial paper | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total cash and cash equivalents	0
Recurring | Commercial paper | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total cash and cash equivalents	1,493
Recurring | Commercial paper | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total cash and cash equivalents	$ 0