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Reclassification out of accumulated other comprehensive income (loss) (Details 2) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Reclassification adjustment out of accumulated other comprehensive income [Line Items]    
Revenues $ 1,042,853 $ 931,604
Other income 2,132 1,242
Interest expense (20,971) (20,874)
Income before income taxes 84,116 84,588
Income taxes 27,932 27,996
Net income (loss) 56,139 56,515
Reclassification adjustment for gain (loss) on derivative instruments included in net income
    
Reclassification adjustment out of accumulated other comprehensive income [Line Items]    
Income before income taxes (548) 4,398
Income taxes 204 (1,626)
Net income (loss) (344) 2,772
Amortization of postretirement liability losses included in net periodic benefit cost
    
Reclassification adjustment out of accumulated other comprehensive income [Line Items]    
Net periodic benefit cost (443) (967)
Income taxes 168 319
Net income (loss) (275) (648)
Reclassification adjustment for loss on available-for-sale investments included in net income
    
Reclassification adjustment out of accumulated other comprehensive income [Line Items]    
Other income (58) (54)
Income taxes 20 19
Net income (loss) (38) (35)
Total reclassifications
    
Reclassification adjustment out of accumulated other comprehensive income [Line Items]    
Net income (loss) (657) 2,089
Commodity derivative instruments [Member] | Reclassification adjustment for gain (loss) on derivative instruments included in net income
    
Reclassification adjustment out of accumulated other comprehensive income [Line Items]    
Revenues (388) 4,513
Interest rate derivative instruments [Member] | Reclassification adjustment for gain (loss) on derivative instruments included in net income
    
Reclassification adjustment out of accumulated other comprehensive income [Line Items]    
Interest expense $ (160) $ (115)