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Components of net periodic benefit cost (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Pension Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service cost $ 1,224 $ 1,270
Interest cost 3,332 3,480
Expected return on plan assets (4,749) (4,816)
Amortization of prior service costs 27 28
Amortization of net loss 1,232 1,043
Net periodic benefit cost 1,066 1,005
Employer contributions 437  
Other Postretirement Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service cost 247 330
Interest cost 1,366 1,456
Amortization of prior service costs (338) (695)
Amortization of net loss 911 876
Net periodic benefit cost $ 2,186 $ 1,967