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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net loss	$ (7,326)	$ (10,537)	$ (21,417)	$ (28,783)
Weighted average shares used to compute per share amount:
Basic (in shares)	46,754	44,665	46,585	44,463
Diluted (in shares)	46,754	44,665	46,585	44,463
Basic net loss per share (USD per share)	$ (0.16)	$ (0.24)	$ (0.46)	$ (0.65)
Diluted net loss per share (USD per share)	$ (0.16)	$ (0.24)	$ (0.46)	$ (0.65)