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Financial Instruments (Changes in Level 3 Inputs) (Details) (USD $)
3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2012
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Hypothetical change to estimated future cash flows attributable to commodity price swaps, percent 10.00%  
Estimated change in fair value resulting from hypothetical percentage change in future cash flows attributable to commodity price swaps   $ 12,000,000
Commodity Contract [Member] | Level 3 [Member]
   
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance, January 1, 2012 (149,278,000) 31,616,000
Unrealized losses included in other comprehensive income 248,572,000 33,553,000
Settlements 20,167,000 54,292,000
Ending balance, March 31, 2012 119,461,000 $ 119,461,000