Stock Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
Stock Plans (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expenses	$ 404	$ 202	$ 1,207	$ 783
Research and development [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expenses	134	85	379	302
General and administrative [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expenses	$ 270	$ 117	$ 828	$ 481