Stockholders' Equity - Summary of Stock Options Outstanding (Detail)	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options Outstanding and Exercisable Number of Shares of common stock subject to options | shares	8,726,092
Options Outstanding and Exercisable Weighted Average Remaining Contractual Life (In years)	7 years 4 months 6 days
Options Exercisable Number of Shares of common stock subject to options | shares	3,699,150
Options Exercisable Weighted Average Exercise Price	$ 9
$2.55 – $3.20 [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Lower Range of Exercise Price	2.55
Upper Range of Exercise Price	$ 3.20
Options Outstanding and Exercisable Number of Shares of common stock subject to options | shares	352,920
Options Outstanding and Exercisable Weighted Average Remaining Contractual Life (In years)	7 years 6 months 21 days
Options Exercisable Number of Shares of common stock subject to options | shares	91,772
Options Exercisable Weighted Average Exercise Price	$ 3
$3.50 – $3.50 [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Lower Range of Exercise Price	3.50
Upper Range of Exercise Price	$ 3.50
Options Outstanding and Exercisable Number of Shares of common stock subject to options | shares	1,171,546
Options Outstanding and Exercisable Weighted Average Remaining Contractual Life (In years)	8 years
Options Exercisable Number of Shares of common stock subject to options | shares	520,209
Options Exercisable Weighted Average Exercise Price	$ 4
$3.55 – $5.70 [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Lower Range of Exercise Price	3.55
Upper Range of Exercise Price	$ 5.70
Options Outstanding and Exercisable Number of Shares of common stock subject to options | shares	894,224
Options Outstanding and Exercisable Weighted Average Remaining Contractual Life (In years)	5 years 21 days
Options Exercisable Number of Shares of common stock subject to options | shares	589,511
Options Exercisable Weighted Average Exercise Price	$ 5
$5.72 – $7.20 [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Lower Range of Exercise Price	5.72
Upper Range of Exercise Price	$ 7.20
Options Outstanding and Exercisable Number of Shares of common stock subject to options | shares	915,176
Options Outstanding and Exercisable Weighted Average Remaining Contractual Life (In years)	7 years 5 months 1 day
Options Exercisable Number of Shares of common stock subject to options | shares	610,840
Options Exercisable Weighted Average Exercise Price	$ 7
$7.70 – $9.99 [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Lower Range of Exercise Price	7.70
Upper Range of Exercise Price	$ 9.99
Options Outstanding and Exercisable Number of Shares of common stock subject to options | shares	897,995
Options Outstanding and Exercisable Weighted Average Remaining Contractual Life (In years)	6 years 2 months 4 days
Options Exercisable Number of Shares of common stock subject to options | shares	566,601
Options Exercisable Weighted Average Exercise Price	$ 9
$10.16 – $12.72 [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Lower Range of Exercise Price	10.16
Upper Range of Exercise Price	$ 12.72
Options Outstanding and Exercisable Number of Shares of common stock subject to options | shares	936,327
Options Outstanding and Exercisable Weighted Average Remaining Contractual Life (In years)	6 years 10 months 28 days
Options Exercisable Number of Shares of common stock subject to options | shares	403,714
Options Exercisable Weighted Average Exercise Price	$ 12
$12.80 – $15.00 [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Lower Range of Exercise Price	12.80
Upper Range of Exercise Price	$ 15.00
Options Outstanding and Exercisable Number of Shares of common stock subject to options | shares	1,213,226
Options Outstanding and Exercisable Weighted Average Remaining Contractual Life (In years)	6 years 4 months 13 days
Options Exercisable Number of Shares of common stock subject to options | shares	694,173
Options Exercisable Weighted Average Exercise Price	$ 14
$15.11 – $19.80 [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Lower Range of Exercise Price	15.11
Upper Range of Exercise Price	$ 19.80
Options Outstanding and Exercisable Number of Shares of common stock subject to options | shares	758,250
Options Outstanding and Exercisable Weighted Average Remaining Contractual Life (In years)	8 years 8 months 26 days
Options Exercisable Number of Shares of common stock subject to options | shares	216,830
Options Exercisable Weighted Average Exercise Price	$ 16
$20.05 – $20.05 [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Lower Range of Exercise Price	20.05
Upper Range of Exercise Price	$ 20.05
Options Outstanding and Exercisable Number of Shares of common stock subject to options | shares	1,003,728
Options Outstanding and Exercisable Weighted Average Remaining Contractual Life (In years)	9 years 21 days
$20.85 – $24.95 [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Lower Range of Exercise Price	$ 20.85
Upper Range of Exercise Price	$ 24.95
Options Outstanding and Exercisable Number of Shares of common stock subject to options | shares	582,700
Options Outstanding and Exercisable Weighted Average Remaining Contractual Life (In years)	9 years 29 days
Options Exercisable Number of Shares of common stock subject to options | shares	5,500
Options Exercisable Weighted Average Exercise Price	$ 21