Note 6 - Share-based Compensation - Stock Option Transactions (Details) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Outstanding options (in shares)			825,251
Outstanding options (in dollars per share)			$ 10.46
Granted options (in shares)	53,970	52,530	155,142	172,862
Granted options (in dollars per share)			$ 18.67
Exercised options (in shares)			(241,844)
Exercised options (in dollars per share)			$ 8.60
Lapsed options (in shares)			(1,100)
Lapsed options (in dollars per share)			$ 5.86
Cancelled options (in shares)			(990)
Cancelled options (in dollars per share)			$ 13.44
Outstanding options (in shares)	736,459		736,459
Outstanding options (in dollars per share)	$ 12.80		$ 12.80