Note 12 - Share-based Compensation - Stock Option Transactions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Outstanding options (in shares)	825,251	946,546
Outstanding options (in dollars per share)	$ 10.46	$ 8.39
Granted options (in shares)	155,142	172,862	153,618
Granted options (in dollars per share)	$ 18.67	$ 16.53
Exercised options (in shares)	(336,726)	(279,766)
Exercised options (in dollars per share)	$ 8.71	$ 7.31
Lapsed options (in shares)	(1,100)	(11,131)
Lapsed options (in dollars per share)	$ 5.86	$ 5.92
Cancelled options (in shares)	(8,690)	(3,260)
Cancelled options (in dollars per share)	$ 15.82	$ 17.53
Outstanding options (in shares)	633,877	825,251	946,546
Outstanding options (in dollars per share)	$ 13.33	$ 10.46	$ 8.39