Stock-Based Compensation - Schedule of Stock Option Activity (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Stock Options Activity [Abstract]
Number of Shares, Outstanding, Beginning balance	12,488,551
Number of Shares, Granted	4,746,250
Number of Shares, Forfeited	(574,773)
Number of Shares, Cancelled	(227,419)
Number of Shares, Exercised	(738,934)
Number of Shares, Outstanding, Ending balance	15,693,675	12,488,551
Weighted Average Exercise Price, Outstanding	$ 2.66
Weighted Average Exercise Price, Granted	1.25
Weighted Average Exercise Price, Forfeited	2.35
Weighted Average Exercise Price, Cancelled	4.16
Weighted Average Exercise Price, Exercised	0.59
Weighted Average Exercise Price, Outstanding	$ 2.32	$ 2.66
Weighted-Average Remaining Contractual Term (Years)	7 years 9 months 10 days	8 years 29 days