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Components of net periodic benefit cost (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Employer contributions     $ 7,419  
Other Postretirement Benefit Plans, Defined Benefit [Member]
        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost 670 370 1,163 895
Interest cost 1,433 1,271 2,756 2,757
Amortization of prior service costs (938) (1,065) (1,998) (2,125)
Amortization of net loss 1,273 1,808 2,776 3,304
Curtailment (20) 0 (20) 0
Total net periodic benefit cost 2,418 2,384 4,677 4,831
Pension Plans
        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost 926 506 1,606 1,401
Interest cost 1,755 1,606 3,390 3,436
Expected return on plan assets (1,893) (1,782) (3,613) (3,482)
Amortization of prior service costs 22 34 54 69
Amortization of net loss 857 1,133 1,706 1,821
Curtailment (18) 0 (18) 0
Total net periodic benefit cost $ 1,649 $ 1,497 $ 3,125 $ 3,245