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Equity (Details 2) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Shares      
Shares under option - beginning of year 684,540 709,300 639,495
Options granted 174,600 148,950 158,300
Options exercised (103,230) (123,150) (80,685)
Options forfeited (24,930) (50,560) (7,810)
Shares under option - end of year 730,980 684,540 709,300
Weighted Average Exercise Price      
Shares under option - beginning of year $ 26.94 $ 24.34 $ 23.19
Options granted 43.48 32.61 23.79
Options exercised 19.03 18.69 13.82
Options forfeited 29.49 27.18 27.77
Shares under option - end of year $ 31.92 $ 26.94 $ 24.34