Investments and Fair Value Measurements- Recurring basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets, Fair Value Disclosure [Abstract]
Investments, amortized cost	$ 24,516	$ 32,608
Recurring [Member]
Assets, Fair Value Disclosure [Abstract]
Total	24,516	33,350
Recurring [Member] | Certificates of Deposit [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents, fair value disclosure	6,146	7,817
Recurring [Member] | Corporate Bonds [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, amortized cost	18,370	25,533
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets, Fair Value Disclosure [Abstract]
Total	24,516	33,350
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Certificates of Deposit [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents, fair value disclosure	6,146	7,817
Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, amortized cost	$ 18,370	$ 25,533