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Components of net periodic benefit cost (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Employer contributions $ 569  
Other Postretirement Benefit Plans, Defined Benefit [Member]
    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service cost 493 525
Interest cost 1,323 1,486
Amortization of prior service costs (1,060) (1,060)
Amortization of net loss 1,503 1,496
Total net periodic benefit cost 2,259 2,447
Pension Plans
    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service cost 680 895
Interest cost 1,635 1,830
Expected return on plan assets (1,720) (1,700)
Amortization of prior service costs 32 35
Amortization of net loss 849 688
Total net periodic benefit cost $ 1,476 $ 1,748