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Components of net periodic benefit cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Pension Benefits [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost $ 2,074 $ 1,333 $ 5,287 $ 3,999
Interest cost 3,348 2,871 10,003 8,612
Expected return on plan assets (5,262) (3,550) (16,025) (10,648)
Amortization of prior service costs 28 9 83 27
Amortization of net loss 1,025 477 3,049 1,430
Curtailment 377 0 365 0
Net periodic benefit cost 1,590 1,140 2,762 3,420
Employer contributions     3,551  
Other Postretirement Benefits [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost 468 397 1,517 1,191
Interest cost 1,542 1,603 4,523 4,809
Amortization of prior service costs (1,007) (961) (2,929) (2,883)
Amortization of net loss 874 925 2,973 2,777
Net periodic benefit cost $ 1,877 $ 1,964 $ 6,084 $ 5,894