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Note 6 - Share-based Compensation - Restricted Share Units (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Number of shares unvested at beginning of period (in shares) 1,242,528 1,502,656 1,516,750
Number of shares unvested at beginning of period, weighted average grant date fair value (in dollars per share) $ 74.71 $ 71.16 $ 63.96
Number of shares granted (in shares) 370,669 158,319 540,746
Number of shares granted, weighted average grant date fair value (in dollars per share) $ 119.82 $ 90.06 $ 75.03
Number of shares vested (in shares) (337,512) (380,702) (520,619)
Number of shares vested, weighted average grant date fair value (in dollars per share) $ 79.02 $ 67.29 $ 54.78
Number of shares forfeited (in shares) (29,850) (37,745) (34,221)
Number of shares forfeited, weighted average grant date fair value (in dollars per share) $ 83.69 $ 75.13 $ 69.14
Number of shares unvested at end of period (in shares) 1,245,835 1,242,528 1,502,656
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share) $ 86.80 $ 74.71 $ 71.16