Stock Option Plan - Summary of Stock Awards and Option Activity Under Stock Incentive Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Awards Available for Grant, Beginning balance	3,933,649
Awards Available for Grant, Authorized	681,660
Awards Available for Grant, Granted	(156,000)
Awards Available for Grant, Canceled	57,501
Awards Available for Grant, Ending balance	4,516,810	3,933,649
Options Issued and Outstanding, Beginning balance	1,573,568
Options Issued and Outstanding, Exercised	(322,465)
Options Issued and Outstanding, Canceled	(57,501)
Options Issued and Outstanding, Ending balance	1,193,602	1,573,568
Options Issued and Outstanding, Options vested and expected to vest	1,193,602
Options Issued and Outstanding, Options vested and exercisable	377,155
Weighted Average Exercise Price, Beginning balance	$ 1.78
Weighted Average Exercise Price, Exercised	1.13
Weighted Average Exercise Price, Canceled	1.62
Weighted Average Exercise Price, Ending balance	1.97	$ 1.78
Weighted Average Exercise Price, Options vested and expected to vest	1.97
Weighted Average Exercise Price, Options vested and exercisable	$ 1.75
Weighted-Average Remaining Contractual Term	7 years 9 months 18 days	8 years 4 months 24 days
Weighted-Average Remaining Contractual Term, Options vested and expected to vest	7 years 9 months 18 days
Weighted-Average Remaining Contractual Term, Options vested and exercisable	7 years 6 months
Aggregate Intrinsic Value	$ 11,066	$ 24,556
Aggregate Intrinsic Value, Options vested and expected to vest	11,066
Aggregate Intrinsic Value, Options vested and exercisable	$ 3,579