Note 6 - Share-based Compensation - Restricted Share Units (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of shares unvested at beginning of period (in shares)	1,245,835	1,242,528	1,502,656
Number of shares unvested at beginning of period, weighted average grant date fair value (in dollars per share)	$ 86.80	$ 74.71	$ 71.16
Number of shares granted (in shares)	440,255	370,669	158,319
Number of shares granted, weighted average grant date fair value (in dollars per share)	$ 99.60	$ 119.82	$ 90.06
Number of shares vested (in shares)	(341,218)	(337,512)	(380,702)
Number of shares vested, weighted average grant date fair value (in dollars per share)	$ 85.61	$ 79.02	$ 67.29
Number of shares forfeited (in shares)	(31,454)	(29,850)	(37,745)
Number of shares forfeited, weighted average grant date fair value (in dollars per share)	$ 93.91	$ 83.69	$ 75.13
Number of shares unvested at end of period (in shares)	1,313,418	1,245,835	1,242,528
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share)	$ 91.22	$ 86.80	$ 74.71