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Components of net periodic benefit cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Pension Benefits [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost $ 968 $ 1,270 $ 2,192 $ 2,540
Interest cost 3,323 3,482 6,655 6,962
Expected return on plan assets (5,116) (4,816) (9,865) (9,632)
Amortization of prior service costs 26 28 53 56
Amortization of net loss 1,506 1,041 2,738 2,084
Net periodic benefit cost 707 1,005 1,773 2,010
Employer contributions     874  
Other Postretirement Benefits [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost 191 330 438 660
Interest cost 2,978 1,439 4,344 2,895
Amortization of prior service costs (698) (695) (1,036) (1,390)
Amortization of net loss (929) 879 (18) 1,755
Net periodic benefit cost $ 1,542 $ 1,953 $ 3,728 $ 3,920