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Components of net periodic benefit cost (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Pension Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service cost $ 1,270 $ 1,704
Interest cost 3,480 3,388
Expected return on plan assets (4,816) (5,494)
Amortization of prior service costs 28 25
Amortization of net loss 1,043 620
Net periodic benefit cost 1,005 243
Employer contributions 464  
Other Postretirement Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service cost 330 562
Interest cost 1,456 1,591
Amortization of prior service costs (695) (961)
Amortization of net loss 876 865
Net periodic benefit cost $ 1,967 $ 2,057