Nature of the business (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ 177,143		$ 173,388
Net loss	$ 3,755	$ 7,517	$ 30,704