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Note 6 - Share-based Compensation - Restricted Share Units (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Number of shares unvested at beginning of period (in shares) 1,516,750 1,940,998 2,421,430
Number of shares unvested at beginning of period, weighted average grant date fair value (in dollars per share) $ 63.96 $ 51.74 $ 41.49
Number of shares granted (in shares) 540,746 390,143 447,780
Number of shares granted, weighted average grant date fair value (in dollars per share) $ 75.03 $ 74.86 $ 75.61
Number of shares vested (in shares) (520,619) (783,483) (808,914)
Number of shares vested, weighted average grant date fair value (in dollars per share) $ 54.78 $ 39.45 $ 37.33
Number of shares forfeited (in shares) (34,221) (30,908) (119,298)
Number of shares forfeited, weighted average grant date fair value (in dollars per share) $ 69.14 $ 54.89 $ 47.81
Number of shares unvested at end of period (in shares) 1,502,656 1,516,750 1,940,998
Number of shares unvested at end of period, weighted average grant date fair value (in dollars per share) $ 71.16 $ 63.96 $ 51.74