Note 5 - Stock-based Compensation - Stock Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares		Dec. 31, 2018 USD ($) $ / shares shares
Outstanding (in shares) | shares	398
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 17.09
Outstanding, weighted average remaining contractual life (Year)	8 years 146 days		8 years 73 days
Outstanding, aggregate intrinsic value | $		[1]	$ 493	[1]
Granted (in shares) | shares	192
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 10.60
Exercised (in shares) | shares
Exercised, weighted average exercise price (in dollars per share) | $ / shares
Forfeited (in shares) | shares
Forfeited, weighted average exercise price (in dollars per share) | $ / shares
Outstanding (in shares) | shares	590		398
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 14.98		$ 17.09
Exercisable (in shares) | shares	239
Exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 14.55
Exercisable, weighted average remaining contractual life (Year)	7 years 146 days
Exercisable, aggregate intrinsic value | $		[1]

[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, 2019 ($10.09) exceeded the exercise price of the underlying options, multiplied by the number of shares subject to each option.