Calculations of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earning per share, Basic and diluted
Net loss (numerator)	$ (16,601)	$ (15,884)	$ (17,199)	$ (20,588)	$ (23,203)	$ (19,338)	$ (22,543)	$ (20,152)	$ (70,272)	$ (85,236)	$ (61,706)
Weighted-average shares, in thousands (denominator)	138,638	138,586	138,426	138,014	137,870	137,375	136,736	136,722	138,418	137,180	136,667
Basic and diluted net loss per share	$ (0.12)	$ (0.12)	$ (0.12)	$ (0.15)	$ (0.17)	$ (0.14)	$ (0.16)	$ (0.15)	$ (0.51)	$ (0.62)	$ (0.45)