Note 3 - Stock-based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Outstanding at beginning of period, shares (in shares)		279
Outstanding at beginning of period, weighted average exercise price (in dollars per share)		$ 15.40
Outstanding, weighted average remaining contractual life (Year)		8 years 146 days	8 years 255 days
Outstanding, aggregate intrinsic value	[1]	$ 956	$ 1,436
Granted, shares (in shares)		119
Granted, weighted average exercise price (in dollars per share)		$ 21.01
Exercised, shares (in shares)
Exercised, weighted average exercise price (in dollars per share)
Forfeited, shares (in shares)
Forfeited, weighted average exercise price (in dollars per share)
Outstanding at end of period, shares (in shares)		398	279
Outstanding at end of period, weighted average exercise price (in dollars per share)		$ 17.09	$ 15.40
Exercisable, shares (in shares)		143
Exercisable, weighted average exercise price (in dollars per share)		$ 14.51
Exercisable, weighted average remaining contractual life (Year)		7 years 292 days
Exercisable, aggregate intrinsic value	[1]	$ 616

[1] The aggregate intrinsic value on this table was calculated based on the amount, if any, by which the closing market value of the Company’s stock on September 30, 2018 ($18.81) exceeded the exercise price of the underlying options, multiplied by the number of shares subject to each option.