Nature of Business, Basis of Presentation and Liquidity - Liquidity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Liquidity
Accumulated deficit	$ 141,019		$ 130,397
Net loss	10,622	$ 9,414
Net cash used in operating activities	(5,478)	$ (6,349)
Cash and cash equivalents	35,376		$ 25,737
Expected Research and development tax incentive payments	$ 1,004
Minimum period to fund Operating expense	12 months