Stock Option Plan - Summary of Stock Awards and Option Activity Under Stock Incentive Plan (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]
Awards Available for Grant, Beginning balance	4,434,310
Share-based compensation arrangement by share-based payment award, additional awards	691,431
Awards Available for Grant, Granted	(30,000)
Awards Available for Grant, Canceled	5,000
Awards Available for Grant, Ending balance	5,100,741	4,434,310
Options Issued and Outstanding, Beginning balance	1,139,935
Options Issued and Outstanding, Exercised	(46,666)
Options Issued and Outstanding, Ending balance	1,093,269	1,139,935
Options Issued and Outstanding, Options vested and expected to vest	1,093,269
Options Issued and Outstanding, Options vested and exercisable	453,169
Weighted Average Exercise Price, Beginning balance	$ 2.01
Weighted Average Exercise Price, Exercised	1.38
Weighted Average Exercise Price, Ending balance	2.03	$ 2.01
Weighted Average Exercise Price, Options vested and expected to vest	2.03
Weighted Average Exercise Price, Options vested and exercisable	$ 1.80
Weighted-Average Remaining Contractual Term	7 years 4 months 24 days	7 years 7 months 6 days
Weighted-Average Remaining Contractual Term, Options vested and expected to vest	7 years 4 months 24 days
Weighted-Average Remaining Contractual Term, Options vested and exercisable	7 years 1 month 6 days
Aggregate Intrinsic Value	$ 5,211	$ 3,428
Aggregate Intrinsic Value, Options vested and expected to vest	5,211
Aggregate Intrinsic Value, Options vested and exercisable	$ 2,265