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Cash and stock-based compensation plans (Details 3) - Stock options [Member]
12 Months Ended
Dec. 31, 2017
USD ($)
NumberofShare
$ / shares
shares
Dec. 31, 2016
USD ($)
NumberofShare
Dec. 31, 2015
USD ($)
NumberofShare
Disclosure Of Sharebased Payment Arrangements [Line Items]      
Number of Share Options Outstanding at Beginning | NumberofShare 485,845 554,756 391,696
Number of share options Granted | NumberofShare 0 0 233,418
Number of share options Forfeited | NumberofShare (69,934) (126) 0
Number of share options Exercised | NumberofShare (142,268) (68,785) (70,358)
Number of Share Options Outstanding at End | NumberofShare 273,643 485,845 554,756
Number of share options Exercisable | NumberofShare 121,840    
Number of share options Expected to vest | shares 151,803    
Weighted average exercise price, Outstanding at Beginning $ 26.87 $ 26.36 $ 23.65
Weighted average exercise price, Granted   0 29.25
Weighted average exercise price, Forfeited 28.63 18.93 0
Weighted average exercise price, Exercised 24.84 22.78 20.86
Weighted average exercise price, Exercisable 27.73    
Weighted average exercise price, Outstanding at End $ 27.48 $ 26.87 $ 26.36
Weighted average exercise price, Exercisable, Expected to vest | $ / shares $ 27.28    
Weighted Average Remaining Contractual Term of Outstanding 3.66 years    
Weighted average remaining contractual term, Exercisable 3.70 years    
Weighted average remaining contractual term, Expected to vest 3.63 years    
Aggregate Intrinsic value, Outstanding at End $ 242    
Aggregate Intrinsic value, Exercisable 115    
Aggregate Intrinsic value, Expected to vest $ 128