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Components of net periodic benefit cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Pension Benefits [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost $ 1,270 $ 2,074 $ 3,810 $ 5,287
Interest cost 3,478 3,348 10,440 10,003
Expected return on plan assets (4,813) (5,262) (14,445) (16,025)
Amortization of prior service costs 28 28 84 83
Amortization of net loss 1,041 1,025 3,125 3,049
Curtailment 0 377 0 365
Net periodic benefit cost 1,004 1,590 3,014 2,762
Employer contributions     1,338  
Other Postretirement Benefits [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost 330 468 990 1,517
Interest cost 1,469 1,542 4,364 4,523
Amortization of prior service costs (695) (1,007) (2,085) (2,929)
Amortization of net loss 878 874 2,633 2,973
Net periodic benefit cost $ 1,982 $ 1,877 $ 5,902 $ 6,084