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Equity (Details 2) - $ / shares
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Shares      
Shares under option - beginning of year 709,300 639,495 643,595
Options granted 149,850 158,300 139,250
Options exercised (123,150) (80,685) (136,640)
Options forfeited (50,560) (7,810) (6,710)
Shares under option - end of year 685,440 709,300 639,495
Weighted Average Exercise Price      
Shares under option - beginning of year $ 24.34 $ 23.19 $ 19.58
Options granted 32.61 23.79 27.93
Options exercised 18.69 13.82 11.19
Options forfeited 27.18 27.77 19.37
Shares under option - end of year $ 26.95 $ 24.34 $ 23.19