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Components of net periodic benefit cost (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Employer contributions $ 3,900  
Other Postretirement Benefit Plans
    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service cost 525 359
Interest cost 1,486 1,386
Expected return on plan assets 0 0
Amortization of prior service costs (1,060) [1] (15,155) [1]
Amortization of net loss 1,496 5,803
Total net periodic benefit cost 2,447 (7,607)
Pension Plans
    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service cost 895 858
Interest cost 1,830 1,680
Expected return on plan assets (1,700) (1,540)
Amortization of prior service costs 35 [1] 35 [1]
Amortization of net loss 688 482
Total net periodic benefit cost $ 1,748 $ 1,515
[1] Plan amendments made in November 2010 resulted in a reduction in OPEB liability and a credit to accumulated other comprehensive loss. The resulting prior service benefit and actuarial losses were amortized ratably into income over the period November 1, 2010 to June 30, 2011 at which time the CAWV OPEB plan terminated.