Organization - Narrative (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2019 USD ($) segment	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of reportable segments | segment					1
Accumulated deficit	$ 391,650				$ 391,650		$ 369,014
Net loss	19,792	$ 2,844	$ 4,651	$ 7,659	22,636	$ 12,310
Cash and cash equivalents	13,962				13,962		18,286
Short-term investments	$ 52,047				$ 52,047		$ 64,638