Organization - Narrative (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of reportable segments | segment									1
Accumulated deficit	$ 378,679				$ 369,014				$ 378,679	$ 369,014
Net loss	(9,501)	$ (3,470)	$ 19,792	$ 2,844	(5,242)	$ 1,069	$ 4,651	$ 7,659	9,665	8,137	$ 17,286
Cash and cash equivalents	26,889				$ 18,286				26,889	$ 18,286
Short term investments	$ 52,100								$ 52,100