Note 6 - Share-based Compensation - Restricted Share Units (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Number of shares unvested at beginning of period (in shares)	1,345,146	1,313,418	1,245,835
Number of shares unvested at beginning of period, weighted average grant date fair value (in dollars per share)	$ 97.22	$ 91.22	$ 86.80
Number of shares granted (in shares)	360,734	511,859	440,255
Number of shares granted, weighted average grant date fair value (in dollars per share)	$ 150.33	$ 110.49	$ 99.60
Number of shares vested (in shares)	(387,948)	(457,437)	(341,218)
Number of shares vested, weighted average grant date fair value (in dollars per share)	$ 100.36	$ 93.78	$ 85.61
Number of shares forfeited (in shares)	(27,700)	(22,694)	(31,454)
Number of shares forfeited, weighted average grant date fair value (in dollars per share)	$ 118.20	$ 102.03	$ 93.91
Number of shares unvested at end of period (in shares)	1,290,232	1,345,146	1,313,418
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share)	$ 110.83	$ 97.22	$ 91.22