Stock-Based Compensation - Summary of Stock Option Activity and Related Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options Outstanding, Options, Balance	405,683
Options Outstanding, Options, Granted	135,972
Options Outstanding, Options, Cancelled	(2,953)
Options Outstanding, Options, Balance	538,702	405,683
Options Outstanding, Options, Exercisable	373,777
Options Outstanding, Weighted-Average Exercise Price, Balance	$ 9.69
Options Outstanding, Weighted-Average Exercise Price, Granted	3.97
Options Outstanding, Weighted-Average Exercise Price, Canceled	12.60
Options Outstanding, Weighted-Average Exercise Price, Balance	8.23	$ 9.69
Options Outstanding, Weighted-Average Exercise Price, Exercisable	$ 9.43
Options Outstanding Weighted-Average Remaining Contractual Life (Years), Balance	7 years 2 months 12 days	6 years 6 months
Options Outstanding Weighted-Average Remaining Contractual Life (Years), Exercisable	6 years 1 month 6 days
Options Outstanding, Aggregate Intrinsic Value, Balance	$ 175