Note 6 - Share-based Payments - Summary of Awards Granted to Key Non-executive Employees (Details) - The 2014 Stock Incentive Plan [Member] - Restricted Stock Units (RSUs) [Member] - Key Employees [Member] - April 8, 2021 [Member]	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Granted on April 10, 2017 [Member]
Vesting percentage	100.00%
Units granted (in shares) | shares	9,995
Grant-date fair value per award (in dollars per share) | $ / shares	$ 32.78
Granted on September 25, 2017 [Member]
Vesting percentage	100.00%
Units granted (in shares) | shares	1,000
Grant-date fair value per award (in dollars per share) | $ / shares	$ 38.29