Note 13 - Stock-based Compensation - Restricted Stock Options (Details) - Restricted Stock [Member] - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Restricted stock (in shares)	307,199	314,973	323,921
Restricted stock, weighted-average grant date fair value (in dollars per share)	$ 98.87	$ 89.55	$ 79.85
Granted (in shares)	205,996	108,179	115,471
Granted, weighted-average grant date fair value (in dollars per share)	$ 123.61	$ 111.76	$ 106.35
Restrictions lapsed (in shares)	(115,908)	(101,941)	(92,597)
Restrictions lapsed, weighted-average grant date fair value (in dollars per share)	$ 91.26	$ 84.70	$ 77.25
Forfeited (in shares)	(53,505)	(14,012)	(31,822)
Forfeited, weighted-average grant date fair value (in dollars per share)	$ 128.05	$ 94.24	$ 87.58
Restricted stock (in shares)	343,782	307,199	314,973
Restricted stock, weighted-average grant date fair value (in dollars per share)	$ 111.79	$ 98.87	$ 89.55