Note 6 - Performance Share Units (Details) - Performance Shares [Member] - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Number of shares unvested at beginning of period (in shares)	433,895	408,025	309,500
Number of shares unvested at beginning of period, weighted average grant date fair value (in dollars per share)	$ 62.23	$ 56.78	$ 47.07
Number of shares granted (in shares)	160,574	106,945	160,500
Number of shares granted, weighted average grant date fair value (in dollars per share)	$ 74.37	$ 76.22	$ 71.90
Number of shares vested (in shares)	(103,796)	(81,075)	(61,975)
Number of shares vested, weighted average grant date fair value (in dollars per share)	$ 58.28	$ 53.26	$ 47.48
Number of shares forfeited (in shares)
Number of shares unvested at end of period (in shares)	490,673	433,895	408,025
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share)	$ 67.04	$ 62.23	$ 56.78