Shareholders' Equity - Summary of Stock Option Activity (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Number of Shares, Beginning Balance	1,390,012
Granted, Number of Shares	575,625
Forfeited, Number of Shares	(54,194)
Expired, Number of Shares	(20,296)
Number of Shares, Ending Balance	1,891,147	1,390,012
Vested and expected to vest, Number of Shares, Ending Balance	1,638,226
Exercisable, Number of Shares, Ending Balance	979,630
Weighted Average Exercise Price, Beginning Balance	$ 4.77
Granted, Weighted Average Exercise Price	1.97
Forfeited, Weighted Average Exercise Price	4.90
Expired, Weighted Average Exercise Price	5.17
Weighted Average Exercise Price, Ending Balance	3.90	$ 4.77
Vested and expected to vest, Weighted Average Exercise Price, Ending Balance	4.13
Exercisable, Weighted Average Exercise Price, Ending Balance	$ 4.85
Weighted Average Remaining Contractual Life (in years)	6 years 11 months 15 days	6 years 9 months 29 days
Vested and expected to vest, Weighted Average Remaining Contractual Life (in years)	6 years 6 months 18 days
Exercisable, Weighted Average Remaining Contractual Life (in years)	4 years
Balance outstanding, Aggregate Intrinsic Value	$ 180
Vested and expected to vest, Aggregate Intrinsic Value	$ 117