Financial Instruments (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Fair value of asset measured on a recurring basis
Fixed income available-for-sale securities		$ 11,960	$ 11,967
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value of asset measured on a recurring basis
Money market funds	[1]	3,576	4,515
Fixed income available-for-sale securities	[2]	0	0
Total		3,576	4,515
Significant Other Observable Inputs (Level 2)
Fair value of asset measured on a recurring basis
Money market funds	[1]	0	0
Fixed income available-for-sale securities	[2]	13,210	13,217
Total		13,210	13,217
Significant Unobservable Inputs (Level 3)
Fair value of asset measured on a recurring basis
Money market funds	[1]	0	0
Fixed income available-for-sale securities	[2]	0	0
Total		0	0
Recurring
Fair value of asset measured on a recurring basis
Money market funds	[1]	3,576	4,515
Fixed income available-for-sale securities	[2]	13,210	13,217
Total		$ 16,786	$ 17,732

[1] Included in cash and cash equivalents in the Company’s condensed consolidated balance sheets

[2] Represents the portfolio of available for sale securities that is included in restricted investments and short-term investments in the Company’s condensed consolidated balance sheets