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Gains and losses on derivatives instruments (Details 2) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Derivative instruments [Line Items]    
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax $ (5,849) $ (3,770)
Amounts reclassified from accumulated other comprehensive gain (loss) 2,772 2,329
Commodity derivatives designated as cash flow hedges [Member]
    
Derivative instruments [Line Items]    
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax (6,154) (4,659)
Amounts reclassified from accumulated other comprehensive gain (loss) 2,843 2,343
Amount of gain (loss) recognized in income statement (ineffective portion), before tax (1,422) (4,251)
Interest rate derivatives designated as cash flow hedges [Member]
    
Derivative instruments [Line Items]    
Amount of gain (loss) recognized in accumulated other comprehensive loss (effective portion), net of tax 305 889
Amounts reclassified from accumulated other comprehensive gain (loss) (71) (14)
Amount of gain (loss) recognized in income statement (ineffective portion), before tax (159) 0
Commodity derivatives not designated as hedging instruments:
    
Derivative instruments [Line Items]    
Amount of gain (loss) recognized in operating revenues, before tax $ (4,410) $ 55