Note 13 - Stock-based Compensation - Stock Option Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Options outstanding (in shares)	252,616	399,055	569,639
Options outstanding, weighted-average exercise price (in dollars per share)	$ 64.71	$ 51.13	$ 45.00
Granted (in shares)	53,708	36,600	60,100
Granted, weighted-average exercise price (in dollars per share)	$ 105.81	$ 110.59	$ 69.06
Exercised (in shares)	(16,079)	(155,765)	(187,313)
Exercised, weighted-average exercise price (in dollars per share)	$ 48.67	$ 38.92	$ 35.93
Cancelled (in shares)	(13,979)	(27,274)	(43,371)
Cancelled, weighted-average exercise price (in dollars per share)	$ 89.50	$ 74.35	$ 61.02
Options outstanding (in shares)	276,266	252,616	399,055
Options outstanding, weighted-average exercise price (in dollars per share)	$ 72.38	$ 64.71	$ 51.13
Options exercisable (in shares)	152,423
Options exercisable, weighted-average exercise price (in dollars per share)	$ 59.26