Net loss per share - Computation of Basic and Diluted Net Income (Loss) per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net loss	$ (14,091)	$ (18,246)
Weighted average shares used to compute per share amount:
Basic (in shares)	46,414	44,259
Diluted (in shares)	46,414	44,259
Basic net loss per share (USD per share)	$ (0.30)	$ (0.41)
Diluted net loss per share (USD per share)	$ (0.30)	$ (0.41)