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Components of net periodic benefit cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Pension Benefits [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost $ 1,509 $ 1,079 $ 3,213 $ 2,666
Interest cost 3,267 3,101 6,655 5,741
Expected return on plan assets (5,269) (3,701) (10,763) (7,098)
Amortization of prior service costs 30 9 55 18
Amortization of net loss 1,405 167 2,024 953
Curtailment (12) 0 (12) 0
Net periodic benefit cost 930 655 1,172 2,280
Employer contributions     2,673  
Other Postretirement Benefits [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost 487 117 1,049 794
Interest cost 1,390 1,695 2,981 3,206
Amortization of prior service costs (961) (961) (1,922) (1,922)
Amortization of net loss 1,234 828 2,099 1,852
Net periodic benefit cost $ 2,150 $ 1,679 $ 4,207 $ 3,930