Note 6 - Share-based Compensation - Summary of Information About Stock Options Outstanding (Details)	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Weighted Average Exercise Price on options (in dollars per share)	$ 12.06
Lower Range of Exercise Prices on options (in dollars per share)	3.82
Upper Range of Exercise Prices on options (in dollars per share)	$ 18.66
Options outstanding (in shares) | shares	720,933
Weighted Average Remaining Contractual Life on options (Year)	3 years 211 days
Exercise Price Range 1 [Member]
Weighted Average Exercise Price on options (in dollars per share)	$ 4.79
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 222 days
Exercise Price Range 2 [Member]
Weighted Average Exercise Price on options (in dollars per share)	$ 6.55
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	231,566
Weighted Average Remaining Contractual Life on options (Year)	1 year 339 days
Exercise Price Range 3 [Member]
Weighted Average Exercise Price on options (in dollars per share)	$ 9.13
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	54,400
Weighted Average Remaining Contractual Life on options (Year)	4 years 193 days
Exercise Price Range 4 [Member]
Weighted Average Exercise Price on options (in dollars per share)	$ 17.25
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	368,967
Weighted Average Remaining Contractual Life on options (Year)	4 years 237 days