Note 3 - Stock-based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Outstanding, December 31, 2017 (in shares)		279
Outstanding, December 31, 2017 (in dollars per share)		$ 15.40
Outstanding, December 31, 2017 (Year)		8 years 255 days	8 years 255 days
Outstanding, December 31, 2017	[1]	$ 1,068	$ 1,436
Granted (in shares)		117
Granted, weighted average exercise price (in dollars per share)		$ 21.04
Exercised (in shares)
Exercised, weighted average exercise price (in dollars per share)
Forfeited (in shares)
Forfeited, weighted average exercise price (in dollars per share)
Outstanding, June 30, 2018 (in shares)		396	279
Outstanding, June 30, 2018 (in dollars per share)		$ 17.07	$ 15.40
Exercisable, June 30, 2018 (in shares)		133
Exercisable, June 30, 2018 (in dollars per share)		$ 14.82
Exercisable, June 30, 2018 (Year)		8 years 36 days
Exercisable, June 30, 2018	[1]	$ 590

[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 June 30, 2018 ($19.24) exceeded the exercise price of the underlying options, multiplied by the number of shares subject to each option.