Organization (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of reportable segments | segment			1
Accumulated deficit	$ 374,256		$ 374,256		$ 360,877
Net loss	1,069	$ 5,407	13,379	$ 17,573
Cash and cash equivalents	53,289		53,289		26,862
Short term investments	$ 44,462		$ 44,462		$ 0