Note 6 - Restricted Share Units (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Number of shares unvested at beginning of period (in shares)	1,940,998	2,421,430	2,835,466
Number of shares unvested at beginning of period, weighted average grant date fair value (in dollars per share)	$ 51.74	$ 41.49	$ 32.75
Number of shares granted (in shares)	390,143	447,780	522,062
Number of shares granted, weighted average grant date fair value (in dollars per share)	$ 74.86	$ 75.61	$ 71.50
Number of shares vested (in shares)	(783,483)	(808,914)	(865,147)
Number of shares vested, weighted average grant date fair value (in dollars per share)	$ 39.45	$ 37.33	$ 31.92
Number of shares forfeited (in shares)	(30,908)	(119,298)	(70,951)
Number of shares forfeited, weighted average grant date fair value (in dollars per share)	$ 54.89	$ 47.81	$ 36.67
Number of shares unvested at end of period (in shares)	1,516,750	1,940,998	2,421,430
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share)	$ 63.96	$ 51.74	$ 41.49