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								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ 2,069	$ 2,272	$ (7,326)	$ (14,091)	$ (6,729)	$ (8,125)	$ (10,537)	$ (18,246)	$ (17,076)	$ (43,637)	$ (53,333)
Denominator:
Weighted average shares used to compute per share amount, basic (in shares)	48,358	47,666	46,754	46,414	46,150	45,476	44,665	44,259	47,304	45,144	43,431
Dilutive effect of equity awards (in shares)									0	0	0
Weighted average shares used to compute diluted net (loss) income (in shares)	50,238	48,615	46,754	46,414	46,150	45,476	44,665	44,259	47,304	45,144	43,431
Basic net loss per share (USD per share)	$ 0.04	$ 0.05	$ (0.16)	$ (0.30)	$ (0.15)	$ (0.18)	$ (0.24)	$ (0.41)	$ (0.36)	$ (0.97)	$ (1.23)
Diluted net loss per share (USD per share)	$ 0.04	$ 0.05	$ (0.16)	$ (0.30)	$ (0.15)	$ (0.18)	$ (0.24)	$ (0.41)	$ (0.36)	$ (0.97)	$ (1.23)