Potential Shares, Excluded from Calculation of Weighted Average Number of Shares Outstanding used for Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of anti-dilutive potential shares	19,874	18,942	20,004	18,646
Stock Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of anti-dilutive potential shares	17,882	17,629	18,137	17,424
Restricted Stock Units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of anti-dilutive potential shares	1,989	1,313	1,865	1,211
Employee Stock Purchase Plan Rights
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of anti-dilutive potential shares	3		2	11