Stock-Based Compensation - Assumptions used in Black-Scholes Option-Pricing Model (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares subject to outstanding common stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk free interest rate, minimum (percent)	1.80%	1.10%	1.50%
Risk free interest rate, maximum (percent)	2.30%	2.10%	1.90%
Expected volatility, minimum (percent)	75.10%	76.50%	76.70%
Expected volatility, maximum (percent)	85.80%	78.10%	79.20%
Expected dividend yield (percent)	0.00%	0.00%	0.00%
Weighted-average fair value of option on grant date (in usd per share)	$ 7.43	$ 6.69	$ 8.37
Shares subject to outstanding common stock options | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	5 years 1 month 6 days	5 years 1 month 6 days	5 years 1 month 6 days
Shares subject to outstanding common stock options | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days
Employee Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk free interest rate, minimum (percent)	1.10%	0.50%	0.10%
Risk free interest rate, maximum (percent)	1.60%	0.80%	0.50%
Expected volatility, minimum (percent)	54.80%	59.50%	67.40%
Expected volatility, maximum (percent)	152.80%	71.30%	77.80%
Expected dividend yield (percent)	0.00%	0.00%	0.00%
Employee Stock [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	6 months	6 months	6 months
Employee Stock [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	1 year	1 year	1 year