Note 6 - Share-based Compensation - Stock Options Outstanding (Details)	3 Months Ended
	Mar. 31, 2019 $ / 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	732,745
Weighted average remaining contractual life on options (Year)	3 years 113 days
Weighted average exercise price on options (in dollars per share)	$ 16.60
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 131 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)	7.96
Upper range of exercise prices on options (in dollars per share)	$ 10.38
Options outstanding (in shares) | shares	42,000
Weighted average remaining contractual life on options (Year)	3 years 182 days
Weighted average exercise price on options (in dollars per share)	$ 8.76
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)	$ 18.86
Options outstanding (in shares) | shares	464,474
Weighted average remaining contractual life on options (Year)	3 years 87 days
Weighted average exercise price on options (in dollars per share)	$ 17.57
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	137,271
Weighted average remaining contractual life on options (Year)	4 years 36 days
Weighted average exercise price on options (in dollars per share)	$ 23.03