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Financial Instruments (Details 3) (Derivatives not designated as hedging instruments [Member], Cash Flow Hedging [Member], USD $)
In Thousands, unless otherwise specified
6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Derivatives not designated as cash-flow hedge instruments    
Amount of gain/loss recognized in income $ 5,439 $ (3,360)
Unrealized gain (loss) on derivative instruments, net [Member] | Natural gas commodity contracts [Member]
   
Derivatives not designated as cash-flow hedge instruments    
Amount of gain/loss recognized in income   (998)
Unrealized gain (loss) on derivative instruments, net [Member] | Crude oil commodity contracts [Member]
   
Derivatives not designated as cash-flow hedge instruments    
Amount of gain/loss recognized in income 5,098 (2,498)
Realized gain (loss) on derivative instruments, net [Member] | Natural gas commodity contracts [Member]
   
Derivatives not designated as cash-flow hedge instruments    
Amount of gain/loss recognized in income   1,972
Realized gain (loss) on derivative instruments, net [Member] | Crude oil commodity contracts [Member]
   
Derivatives not designated as cash-flow hedge instruments    
Amount of gain/loss recognized in income $ 341 $ (1,836)