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Acquisitions (Summary Of Fair Value Of Assets Aquired And Liabilities Assumed) (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Jun. 30, 2012
Prize [Member]
Apr. 20, 2012
Prize [Member]
Business Acquisition [Line Items]        
Evaluated $ 1,259,868 $ 975,182   $ 223,740
Unevaluated 9,000     9,000
Asset retirement obligation       (4,738)
Environmental liability     (1,900) (1,891)
Other current liabilities       (993)
Net assets acquired       $ 225,118