Note 6 - Share-based Compensation - Restricted Share Units (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of shares unvested at beginning of period (in shares)	1,502,656	1,516,750	1,940,998
Number of shares unvested at beginning of period, weighted average grant date fair value (in dollars per share)	$ 71.16	$ 63.96	$ 51.74
Number of shares granted (in shares)	158,319	540,746	390,143
Number of shares granted, weighted average grant date fair value (in dollars per share)	$ 90.06	$ 75.03	$ 74.86
Number of shares vested (in shares)	(380,702)	(520,619)	(783,483)
Number of shares vested, weighted average grant date fair value (in dollars per share)	$ 67.29	$ 54.78	$ 39.45
Number of shares forfeited (in shares)	(37,745)	(34,221)	(30,908)
Number of shares forfeited, weighted average grant date fair value (in dollars per share)	$ 75.13	$ 69.14	$ 54.89
Number of shares unvested at end of period (in shares)	1,242,528	1,502,656	1,516,750
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share)	$ 74.71	$ 71.16	$ 63.96