Note 5 - Stock-based Compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2019	Dec. 31, 2018
Outstanding (in shares)		398
Granted (in shares)		192
Exercised (in shares)
Forfeited (in shares)
Outstanding (in shares)		590	398
Exercisable (in shares)		239
Outstanding, weighted average exercise price (in dollars per share)		$ 17.09
Granted, weighted average exercise price (in dollars per share)		10.60
Exercised, weighted average exercise price (in dollars per share)
Forfeited, weighted average exercise price (in dollars per share)
Outstanding, weighted average exercise price (in dollars per share)		14.98	$ 17.09
Exercisable, weighted average exercise price (in dollars per share)		$ 14.55
Outstanding, weighted average remaining contractual life (Year)	[1]	8 years 36 days	8 years 73 days
Exercisable, weighted average remaining contractual life (Year)	[1]	7 years 73 days
Outstanding, aggregate intrinsic value	[1]		$ 493
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 September 30, 2019 ($9.11) exceeded the exercise price of the underlying options, multiplied by the number of shares subject to each option.