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Financial Instruments (Details 3) (Derivatives not designated as cash-flow hedging instruments [Member], Cash Flow Hedging [Member], USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Derivatives not designated as cash-flow hedge instruments:    
Amount of gain/loss recognized in income $ 3,985 $ 6,687
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 1,040 (3,037)
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 2,443 2,123
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   6,289
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 $ 502 $ 1,312