Financial Instruments (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Fair value of asset measured on a recurring basis
Fixed income available-for-sale securities		$ 11,967	$ 11,983
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair value of asset measured on a recurring basis
Money market funds	[1]	4,515	4,513
Fixed income available-for-sale securities	[2]	0	0
Assets, Fair Value Disclosure, Recurring		4,515	4,513
Significant Other Observable Inputs (Level 2) [Member]
Fair value of asset measured on a recurring basis
Money market funds	[1]	0	0
Fixed income available-for-sale securities	[2]	13,217	13,233
Assets, Fair Value Disclosure, Recurring		13,217	13,233
Significant Unobservable Inputs (Level 3) [Member]
Fair value of asset measured on a recurring basis
Money market funds	[1]	0	0
Fixed income available-for-sale securities	[2]	0	0
Assets, Fair Value Disclosure, Recurring		0	0
Fair Value, Measurements, Recurring [Member]
Fair value of asset measured on a recurring basis
Money market funds	[1]	4,515	4,513
Fixed income available-for-sale securities	[2]	13,217	13,233
Assets, Fair Value Disclosure, Recurring		$ 17,732	$ 17,746

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

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