Net loss per share - Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	6,629	5,584	4,241
Outstanding stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	5,483	4,643	3,755
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	1,127	931	477
ESPP shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the calculation of diluted net loss per share (in shares)	19	10	9