Note 6 - Share-based Compensation - Stock Options Outstanding (Details)	9 Months Ended
	Sep. 30, 2017 $ / 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)	$ 21.63
Options outstanding (in shares) | shares	736,459
Weighted average remaining contractual life on options (Year)	3 years 302 days
Weighted average exercise price on options (in dollars per share)	$ 12.80
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)	$ 5.55
Options outstanding (in shares) | shares	66,000
Weighted average remaining contractual life on options (Year)	2 years 131 days
Weighted average exercise price on options (in dollars per share)	$ 4.79
Exercise Price Range 2 [Member]
Lower range of exercise prices on options (in dollars per share)	5.65
Upper range of exercise prices on options (in dollars per share)	$ 7.36
Options outstanding (in shares) | shares	213,112
Weighted average remaining contractual life on options (Year)	1 year 284 days
Weighted average exercise price on options (in dollars per share)	$ 6.55
Exercise Price Range 3 [Member]
Lower range of exercise prices on options (in dollars per share)	7.96
Upper range of exercise prices on options (in dollars per share)	$ 10.38
Options outstanding (in shares) | shares	50,400
Weighted average remaining contractual life on options (Year)	4 years 171 days
Weighted average exercise price on options (in dollars per share)	$ 9.03
Exercise Price Range 4 [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)	$ 21.63
Options outstanding (in shares) | shares	406,947
Weighted average remaining contractual life on options (Year)	5 years 25 days
Weighted average exercise price on options (in dollars per share)	$ 17.84