Stock-Based Compensation - Schedule of Stock Option Activity (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Stock Options Activity [Abstract]
Number of Shares, Outstanding, Beginning balance	15,693,675
Number of Shares, Granted	8,705,617	4,746,250	5,368,755
Number of Shares, Forfeited	(925,495)
Number of Shares, Cancelled	(192,633)
Number of Shares, Exercised	(3,381,533)
Number of Shares, Outstanding, Ending balance	19,899,631	15,693,675
Weighted Average Exercise Price, Outstanding	$ 2.32
Weighted Average Exercise Price, Granted	2.39
Weighted Average Exercise Price, Forfeited	1.59
Weighted Average Exercise Price, Cancelled	2.11
Weighted Average Exercise Price, Exercised	2.11
Weighted Average Exercise Price, Outstanding	$ 2.42	$ 2.32
Weighted-Average Remaining Contractual Term (Years)	7 years 9 months 25 days	7 years 9 months 10 days