Stockholders' Equity - Summary of Employee Stock Options Outstanding (Detail) - Stock Option	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Shares (in shares) | shares	2,933,232
Options Outstanding, Weighted Average Remaining Contractual Term	7 years 6 months
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 3.82
Options Exercisable, Shares (in shares) | shares	408,042
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 5.07
Range One
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price, Lower Range (in dollars per share)	0
Exercise Price, Upper Range (in dollars per share)	$ 5
Options Outstanding, Shares (in shares) | shares	2,801,699
Options Outstanding, Weighted Average Remaining Contractual Term	7 years 9 months 8 days
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 3.61
Options Exercisable, Shares (in shares) | shares	276,509
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 3.57
Range Two
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price, Lower Range (in dollars per share)	5.01
Exercise Price, Upper Range (in dollars per share)	$ 10
Options Outstanding, Shares (in shares) | shares	105,783
Options Outstanding, Weighted Average Remaining Contractual Term	1 year 4 months 17 days
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 6.76
Options Exercisable, Shares (in shares) | shares	105,783
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 6.76
Range Three
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise Price, Lower Range (in dollars per share)	10.01
Exercise Price, Upper Range (in dollars per share)	$ 15
Options Outstanding, Shares (in shares) | shares	25,750
Options Outstanding, Weighted Average Remaining Contractual Term	2 years 10 months 28 days
Options Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 14.30
Options Exercisable, Shares (in shares) | shares	25,750
Options Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 14.30