Note 6 - Share-based Compensation - Stock Options Outstanding (Details)	3 Months Ended
	Mar. 31, 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)	$ 23.59
Options outstanding (in shares) | shares	681,654
Weighted average remaining contractual life on options (Year)	3 years 116 days
Weighted average exercise price on options (in dollars per share)	$ 14.81
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	102,800
Weighted average remaining contractual life on options (Year)	2 years 339 days
Weighted average exercise price on options (in dollars per share)	$ 5.26
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	128,367
Weighted average remaining contractual life on options (Year)	2 years 14 days
Weighted average exercise price on options (in dollars per share)	$ 7.88
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	316,017
Weighted average remaining contractual life on options (Year)	3 years 146 days
Weighted average exercise price on options (in dollars per share)	$ 17.28
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)	$ 23.59
Options outstanding (in shares) | shares	134,470
Weighted average remaining contractual life on options (Year)	4 years 240 days
Weighted average exercise price on options (in dollars per share)	$ 22.90