Note 12 - Stock-based Compensation - Stock Option Activity (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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
Granted (in shares)	60,065	53,708	36,600
Granted, weighted-average exercise price (in dollars per share)	$ 105.79	$ 105.81	$ 110.59
Exercised (in shares)	(68,958)	(16,079)	(155,765)
Exercised, weighted-average exercise price (in dollars per share)	$ 57.00	$ 48.67	$ 38.92
Cancelled (in shares)	(37,842)	(13,979)	(27,274)
Cancelled, weighted-average exercise price (in dollars per share)	$ 67.92	$ 89.50	$ 74.35
Options outstanding (in shares)	229,531	276,266	252,616
Options outstanding, weighted-average exercise price (in dollars per share)	$ 86.46	$ 72.38	$ 64.71
Options exercisable (in shares)	94,318
Options exercisable, weighted-average exercise price (in dollars per share)	$ 69.36