Note 5 - Share-based Compensation - Performance Share Units (Details) - Performance Shares [Member] - $ / shares	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Number of shares unvested at beginning of period (in shares)	374,010	333,925	375,528
Number of shares unvested at beginning of period, weighted average grant date fair value (in dollars per share)	$ 123.42	$ 109.57	$ 95.67
Number of shares granted (in shares)	135,842	135,500	202,023
Number of shares granted, weighted average grant date fair value (in dollars per share)	$ 189.05	$ 143.32	$ 112.87
Number of shares vested (in shares)	(108,823)	(95,415)	(145,038)
Number of shares vested, weighted average grant date fair value (in dollars per share)	$ 117.57	$ 103.21	$ 89.75
Number of shares forfeited (in shares)	0	0	(98,588)
Number of shares forfeited, weighted average grant date fair value (in dollars per share)	$ 0	$ 0	$ 110.19
Number of shares unvested at end of period (in shares)	401,029	374,010	333,925
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share)	$ 146.96	$ 123.42	$ 109.57