Stock Option Plan - Summary of stock-based compensation activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Number of Stock Options Outstanding
Balance, January 1, 2018	1,855,138
Options granted	135,000
Options exercised	(216,193)
Options forfeited/canceled	(6,410)
Balance, September 30, 2018	1,768,012	1,855,138
Exercisable as of September 30, 2018	1,452,400
Weighted Average Exercise Price
Balance, January 1, 2018	$ 2.69
Options granted	8.00
Options exercised	2.03
Options forfeited/canceled	3.44
Balance, September 30, 2018	3.17	$ 2.69
Exercisable as of September 30, 2018	$ 2.83
Weighted Average Remaining Contractual Term, Options Outstanding (in years)	7 years 1 month 2 days	7 years 5 months 23 days
Weighted Average Remaining Contractual Term, Options Exercisable (in years)	6 years 9 months 22 days
Aggregate Intrinsic Value, Options Outstanding	$ 57,330	$ 9,850
Aggregate Intrinsic Value, Options Exercisable	$ 47,589

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.