Stock Options - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Options
Outstanding at beginning of the year (in shares)	2,071,616	1,497,511	1,541,163
Granted (in shares)	1,014,556	721,763	120,277
Exercised (in shares)	(301,058)	(127,122)	(109,079)
Forfeited (in shares)	(39,958)	(20,536)	(54,850)
Outstanding at ending of the year (in shares)	2,745,156	2,071,616	1,497,511	1,541,163
Exercisable (in shares)	1,297,075
Weighted Average Exercise Price
Outstanding, beginning of the period (in dollars per share)	$ 1.47	$ 1.76	$ 1.77
Granted (in dollars per share)	31.91	0.94	1.65
Exercised (in dollars per share)	1.86	1.85	1.74
Forfeited (in dollars per share)	3.64	1.88	1.87
Outstanding, end of the period (in dollars per share)	12.64	$ 1.47	$ 1.76	$ 1.77
Exercisable (in dollars per share)	$ 1.54
Weighted average remaining contractual term
Outstanding (in years)	7 years 4 months 24 days	5 years 10 months 24 days	5 years 9 months 18 days	6 years 7 months 6 days
Exercisable	5 years 3 months 18 days
Aggregate intrinsic value
Outstanding	$ 81,453
Exercisable	52,801
Fair value of options vested	$ 200	$ 100	$ 100