Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares, Outstanding at beginning balance		3,351,132
Shares, Granted	0	4,622,428
Shares, Expired/cancelled		(1,247,463)
Shares, Outstanding at ending balance	6,726,097	6,726,097	3,351,132
Shares, Expected to vest	5,049,655	5,049,655
Shares, Options exercisable	1,676,442	1,676,442
Weighted Average Exercise Price, Outstanding at beginning balance		$ 3.73
Weighted Average Exercise Price, Granted		1.86
Weighted Average Exercise Price, Expired/cancelled		2.77
Weighted Average Exercise Price, Outstanding at ending balance	$ 2.62	2.62	$ 3.73
Weighted Average Exercise Price, Expected to vest	2.32	2.32
Weighted Average Exercise Price, Options exercisable	$ 3.55	$ 3.55
Weighted Average Remaining Contractual Term, Outstanding		8 years 5 months 1 day	8 years 2 months 26 days
Weighted Average Remaining Contractual Term, Expected to vest		2 years 4 months 24 days
Weighted Average Remaining Contractual Term, Options exercisable		6 years 3 months 21 days
Aggregate Intrinsic Value, Outstanding			$ 142,788