Net loss per share - Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	6,527	5,372
Outstanding Stock Options [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	5,384	4,446
Restricted Stock Units [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	1,128	920
ESPP Shares [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share	15	6