Calculations of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Earning per share, Basic and diluted
Net loss (numerator)	$ (19,583)	$ (19,426)	$ (18,959)	$ (15,884)	$ (17,199)	$ (20,588)	$ (57,968)	$ (53,671)
Weighted-average shares, in thousands (denominator)	139,589			138,586			139,025	138,344
Basic and diluted net loss per share	$ (0.14)			$ (0.12)			$ (0.42)	$ (0.39)