Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Total investments	$ 69,489	$ 17,060
Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	19,514	5,221
Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	49,975	11,839
Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
Money market funds
Investment [Line Items]
Total investments	10,970	5,221
Money market funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	10,970	5,221
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	7,257
Certificates of deposit | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	0
Certificates of deposit | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	7,257
Certificates of deposit | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0
US Treasury securities
Investment [Line Items]
Total investments	8,544
US Treasury securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	8,544
US Treasury securities | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	0
US Treasury securities | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0
US Agency securities
Investment [Line Items]
Total investments	4,501
US Agency securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	0
US Agency securities | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	4,501
US Agency securities | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0
Asset-backed securities
Investment [Line Items]
Total investments	5,557	2,507
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	5,557	2,507
Asset-backed securities | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
Corporate notes and bonds
Investment [Line Items]
Total investments	32,660	9,332
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	32,660	9,332
Corporate notes and bonds | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	0	0
Total investments
Investment [Line Items]
Total investments	58,519	11,839
Total investments | Quoted Prices in Active Markets for Identical Assets (Level 1)
Investment [Line Items]
Total investments	8,544	0
Total investments | Significant Other Observable Inputs (Level 2)
Investment [Line Items]
Total investments	49,975	11,839
Total investments | Significant Unobservable Inputs (Level 3)
Investment [Line Items]
Total investments	$ 0	$ 0