Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Total investments	$ 88,480	$ 69,489
Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	22,548	19,514
Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	65,932	49,975
Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
Money market funds
Investment [Line Items]
Total investments	7,832	10,970
Money market funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	7,832	10,970
Money market funds | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	0	0
Money market funds | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
Certificates of deposit
Investment [Line Items]
Total investments	13,441	7,257
Certificates of deposit | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	0	0
Certificates of deposit | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	13,441	7,257
Certificates of deposit | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
US Treasury securities
Investment [Line Items]
Total investments	14,716	8,544
US Treasury securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	14,716	8,544
US Treasury securities | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	0	0
US Treasury securities | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
US Agency securities
Investment [Line Items]
Total investments	8,459	4,501
US Agency securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	0	0
US Agency securities | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	8,459	4,501
US Agency securities | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
Commercial paper
Investment [Line Items]
Total investments	9,171
Commercial paper | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	0
Commercial paper | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	9,171
Commercial paper | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0
Asset-backed securities
Investment [Line Items]
Total investments	3,025	5,557
Asset-backed securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	0	0
Asset-backed securities | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	3,025	5,557
Asset-backed securities | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
Corporate notes and bonds
Investment [Line Items]
Total investments	31,836	32,660
Corporate notes and bonds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	0	0
Corporate notes and bonds | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	31,836	32,660
Corporate notes and bonds | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
Total investments
Investment [Line Items]
Total investments	80,648	58,519
Total investments | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	14,716	8,544
Total investments | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	65,932	49,975
Total investments | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	$ 0	$ 0