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Note 6 - Share-based Compensation - Performance Share Units (Details) - Performance Shares [Member] - $ / shares
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Number of shares unvested at beginning of period (in shares) 490,673 433,895 408,025
Number of shares unvested at beginning of period, weighted average grant date fair value (in dollars per share) $ 67.04 $ 62.23 $ 56.78
Number of shares granted (in shares) 142,114 160,574 106,945
Number of shares granted, weighted average grant date fair value (in dollars per share) $ 74.71 $ 74.37 $ 76.22
Number of shares vested (in shares) (148,733) (103,796) (81,075)
Number of shares vested, weighted average grant date fair value (in dollars per share) $ 62.84 $ 58.28 $ 53.26
Number of shares forfeited (in shares)
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share)
Number of shares unvested at end of period (in shares) 484,054 490,673 433,895
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share) $ 70.58 $ 67.04 $ 62.23