Note 6 - Share-based Compensation - Performance Share Units (Details) - Performance Shares [Member] - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Number of shares unvested at beginning of period (in shares)	333,925	375,528	360,512
Number of shares unvested at beginning of period, weighted average grant date fair value (in dollars per share)	$ 109.57	$ 95.67	$ 93.74
Number of shares granted (in shares)	135,500	202,023	142,156
Number of shares granted, weighted average grant date fair value (in dollars per share)	$ 143.32	$ 112.87	$ 98.58
Number of shares vested (in shares)	(95,415)	(145,038)	(127,140)
Number of shares vested, weighted average grant date fair value (in dollars per share)	$ 103.21	$ 89.75	$ 93.46
Number of shares forfeited (in shares)	0	(98,588)	0
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share)	$ 0	$ 110.19	$ 0
Number of shares unvested at end of period (in shares)	374,010	333,925	375,528
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share)	$ 123.42	$ 109.57	$ 95.67