Warrants - Summary of Warrant Activity (Detail) - Warrants Not Settleable in Cash [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Warrant or Right [Line Items]
Beginning balance	1,426,622	1,426,622	1,313,719
Number of Warrants, Granted	45,769,746		112,903
Number of Warrants, Exercised	(34,033,700)
Ending balance	13,162,668	1,426,622	1,426,622	1,313,719
Weighted Average Exercise Price, Outstanding, Beginning balance	$ 1.81	$ 1.81	$ 1.70
Weighted Average Exercise Price, Granted	1.00		3.10
Weighted Average Exercise Price, Exercised	1.00
Weighted Average Exercise Price, Outstanding, Ending balance	$ 1.08	$ 1.81	$ 1.81	$ 1.70
Weighted Average Remaining Contractual Life, Outstanding	4 years 6 months	2 years 2 months 12 days	3 years 2 months 12 days	3 years 10 months 24 days
Weighted Average Remaining Contractual Life, Granted	4 years 9 months 18 days		6 years 7 months 6 days
Weighted Average Fair Value, Outstanding, Beginning balance	$ 1.54	$ 1.54	$ 1.75
Weighted Average Fair Value, Granted	0.22		0.86
Weighted Average Fair Value, Outstanding, Ending balance	$ 0.39	$ 1.54	$ 1.54	$ 1.75