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Cash and stock-based compensation plans (Details 1) - Key management personnel of entity or parent [member] - Restricted Stocks [Member]
 12 Months Ended
Dec. 31, 2017
USD ($)
NumberofShare
Dec. 31, 2016
USD ($)
NumberofShare
Dec. 31, 2015
USD ($)
NumberofShare
Disclosure Of Sharebased Payment Arrangements [Line Items]      
Number of Shares, Outstanding at Beginning | NumberofShare 167,436 162,748 163,712
Number of Shares, Granted | NumberofShare 25,289 91,454 63,244
Number of Shares, Forfeited | NumberofShare (71,401) (21,408) 0
Number of Shares, Vested | NumberofShare (70,519) (65,358) (64,208)
Number of Shares, Outstanding at End | NumberofShare 50,805 167,436 162,748
Number of Shares, Expected to vest | NumberofShare 50,805    
Weighted average grand date fair value Outstanding at beginning $ 19.35 $ 19.74 $ 18.18
Weighted average grand date fair value, Granted 25.70 18.26 21.67
Weighted average grand date fair value, Forfeited 18.61 17.69  
Weighted average grand date fair value, Vested 19.76 18.83 17.67
Weighted average grand date fair value, Outstanding at End $ 21.07 $ 19.35 $ 19.74
Weighted average remaining contractual term, Outstanding 2.02 years    
Weighted average grand date fair value, Expected to vest $ 21.07    
Weighted average remaining contractual term, Expected to vest 2.02 years    
Aggregate Intrinsic value, Outstanding $ 518    
Aggregate Intrinsic value, Expected to vest $ 296