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Derivative Instruments (Schedule Of Location And Amounts Of Realized And Unrealized Gains And Losses Of Commodity Derivative Contracts) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Derivative Instruments [Abstract]      
Change in unrealized gain (loss) on derivative instruments $ 5,436 $ (7,533) $ (7,043)
Realized gain (loss) on derivative instruments (3,841) (120) 2,296
Total net gain (loss) on derivative instruments $ 1,595 $ (7,653) $ (4,747)