Note 6 - Share-based Compensation - Stock Options Outstanding (Details)	9 Months Ended
	Sep. 30, 2018 $ / shares shares
Lower range of exercise prices on options (in dollars per share)	$ 3.82
Upper range of exercise prices on options (in dollars per share)	$ 24.28
Options outstanding (in shares) | shares	711,029
Weighted average remaining contractual life on options (Year)	3 years 43 days
Weighted average exercise price on options (in dollars per share)	$ 15.36
Exercise Price Range 1 [Member]
Lower range of exercise prices on options (in dollars per share)	3.82
Upper range of exercise prices on options (in dollars per share)	$ 5.88
Options outstanding (in shares) | shares	89,000
Weighted average remaining contractual life on options (Year)	2 years 313 days
Weighted average exercise price on options (in dollars per share)	$ 5.33
Exercise Price Range 2 [Member]
Lower range of exercise prices on options (in dollars per share)	7.36
Upper range of exercise prices on options (in dollars per share)	$ 10.38
Options outstanding (in shares) | shares	126,767
Weighted average remaining contractual life on options (Year)	1 year 200 days
Weighted average exercise price on options (in dollars per share)	$ 7.85
Exercise Price Range 3 [Member]
Lower range of exercise prices on options (in dollars per share)	16.35
Upper range of exercise prices on options (in dollars per share)	$ 18.66
Options outstanding (in shares) | shares	352,232
Weighted average remaining contractual life on options (Year)	3 years 54 days
Weighted average exercise price on options (in dollars per share)	$ 17.49
Exercise Price Range 4 [Member]
Lower range of exercise prices on options (in dollars per share)	21.63
Upper range of exercise prices on options (in dollars per share)	$ 24.28
Options outstanding (in shares) | shares	143,030
Weighted average remaining contractual life on options (Year)	4 years 211 days
Weighted average exercise price on options (in dollars per share)	$ 23.03