Net loss per share attributable to common stockholders - Calculation of basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net loss	$ (2,554)	$ (8,501)
Weighted average shares outstanding (in shares)	5,123,568	4,928,881
Basic and diluted loss per share (in dollars per share)	$ (0.50)	$ (1.72)