Note 6 - Share-based Compensation - Stock Options Outstanding (Details)	6 Months Ended
	Jun. 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	672,854
Weighted average remaining contractual life on options (Year)	3 years 91 days
Weighted average exercise price on options (in dollars per share)	$ 15.12
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	90,800
Weighted average remaining contractual life on options (Year)	3 years 18 days
Weighted average exercise price on options (in dollars per share)	$ 5.34
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	127,767
Weighted average remaining contractual life on options (Year)	1 year 292 days
Weighted average exercise price on options (in dollars per share)	$ 7.87
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	310,187
Weighted average remaining contractual life on options (Year)	3 years 62 days
Weighted average exercise price on options (in dollars per share)	$ 17.30
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	144,100
Weighted average remaining contractual life on options (Year)	4 years 302 days
Weighted average exercise price on options (in dollars per share)	$ 23.02