Reconciliation of Numerator and Denominator Used in Computation of Basic and Diluted Net Income Loss per Share (Detail) - 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 for Basic and Diluted:
Net loss used for basic calculation	$ (16,923)	$ (17,967)	$ (17,562)	$ (18,792)	$ (16,205)	$ (14,192)	$ (13,282)	$ (13,885)	$ (71,244)	$ (57,564)	$ (60,585)
Denominator:
Basic weighted average number of shares outstanding									139,831	131,663	108,221
Diluted weighted average number of shares outstanding									139,831	131,663	108,221
Net loss per share:
Basic and diluted									$ (0.51)	$ (0.44)	$ (0.56)