Stock-Based Compensation - Summary of Stock Option Activity (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares, Outstanding at beginning balance | shares	1,912,980
Shares, Granted | shares	1,612,700
Shares, Exercised | shares	(52,099)
Shares, Expired/cancelled | shares	(124,949)
Shares, Outstanding at ending balance | shares	3,348,632	1,912,980
Shares, Expected to vest | shares	2,436,229
Shares, Options exercisable | shares	912,403
Weighted Average Exercise Price, Outstanding at beginning balance | $ / shares	$ 3.72
Weighted Average Exercise Price, Granted | $ / shares	3.64
Weighted Average Exercise Price, Exercised | $ / shares	1.47
Weighted Average Exercise Price, Expired/cancelled | $ / shares	3.23
Weighted Average Exercise Price, Outstanding at ending balance | $ / shares	3.73	$ 3.72
Weighted Average Exercise Price, Expected to vest | $ / shares	4.27
Weighted Average Exercise Price, Options exercisable | $ / shares	$ 2.28
Weighted Average Remaining Contractual Term, Outstanding	8 years 5 months 19 days	8 years 2 months 26 days
Weighted Average Remaining Contractual Term, Expected to vest	9 years 4 months 28 days
Weighted Average Remaining Contractual Term, Options exercisable	5 years 11 months 15 days
Aggregate Intrinsic Value, Outstanding | $	$ 1,129,828	$ 2,738,704
Aggregate Intrinsic Value, Expected to vest | $	61,555
Aggregate Intrinsic Value, Options exercisable | $	$ 1,068,273