Note 7 - Stock-based Compensation - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) $ / shares shares
Outstanding at January 1, 2020 (in shares) | shares	1,541,792
Outstanding at January 1, 2020, weighted average exercise price (in dollars per share) | $ / shares	$ 6.88
Granted (in shares) | shares
Granted, weighted average exercise price (in dollars per share) | $ / shares
Exercised (in shares) | shares	(7,633)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 3.75
Forfeited or expired (in shares) | shares	(4,751)
Forfeited or expired, weighted average exercise price (in dollars per share) | $ / shares	$ 8.12
Outstanding at March 31, 2020 (in shares) | shares	1,529,408
Outstanding at March 31, 2020, weighted average exercise price (in dollars per share) | $ / shares	$ 6.89
Outstanding at March 31, 2020, weighted average remaining contractual term (Year)	2 years 357 days
Outstanding at March 31, 2020, aggregate intrinsic value | $	$ 478
Vested and Expected to Vest (in shares) | shares	1,438,369
Options vested and expected to vest, weighted average exercise price (in dollars per share) | $ / shares	$ 6.80
Options vested and expected to vest, weighted average remaining contractual term (Year)	2 years 295 days
Options vested and expected to vest, aggregate intrinsic value | $	$ 478
Exercisable at March 31, 2020 (in shares) | shares	1,092,278
Exercisable at March 31, 2020, weighted average exercise price (in dollars per share) | $ / shares	$ 6.29
Exercisable at March 31, 2020, weighted average remaining contractual term (Year)	1 year 324 days
Exercisable at March 31, 2020, aggregate intrinsic value | $	$ 478