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Cash and stock-based compensation plans (Details) - Directors [Member] - Restricted Stocks [Member]
 12 Months Ended
Dec. 31, 2017
USD ($)
NumberofShare
shares
Dec. 31, 2016
USD ($)
NumberofShare
Dec. 31, 2015
USD ($)
NumberofShare
Disclosure Of Sharebased Payment Arrangements [Line Items]      
Number of Shares, Outstanding at Beginning | NumberofShare 96,900 96,321 78,336
Number of Shares, Granted | NumberofShare 57,000 57,000 57,000
Number of Shares, Vested | NumberofShare (61,950) (56,421) (39,015)
Number of Shares, Outstanding at End | NumberofShare 91,950 96,900 96,321
Number of Shares, Expected to vest | shares 91,950    
Weighted average grand date fair value Outstanding at beginning | $ $ 27.86 $ 30.62 $ 24.37
Weighted average grand date fair value, Granted | $ 27.80 24.14 33.78
Weighted average grand date fair value, Vested | $ 28.50 28.80 22.69
Weighted average grand date fair value, Outstanding at End | $ $ 27.40 $ 27.86 $ 30.62