Net Loss Per Share - Instruments Excluded from Diluted Net Loss Per Share (Detail) - shares shares in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	6,148	6,620
Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	5,451	6,020
Warrants to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	107	100
Restricted Stock and Performance Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	562	459
ESPP [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total shares	28	41