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           Supplemental Oil And Gas Disclosures (Capitalized Costs Not Subject To Amortization) (Details) (USD $) 
        In Millions, unless otherwise specified  | 
        12 Months Ended | |||
|---|---|---|---|---|
| 
           Dec. 31, 2011 
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           Dec. 31, 2010 
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           Dec. 31, 2009 
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           Dec. 31, 2008 
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      |
| Supplemental Oil And Gas Disclosures [Abstract] | ||||
| Acquisition costs | $ 81.3 | $ 44.4 | ||
| Capitalized interest not subject to amortization | 9.6 | 4.8 | 4.2 | 10.2 | 
| Total costs not subject to amortization | 90.9 | 4.8 | 4.2 | 54.6 | 
| Acquisition costs, Total | 125.7 | |||
| Capitalized interest not subject to amortization, Total | 28.8 | |||
| Total costs not subject to amortization, Total | $ 154.5 | |||
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- Definition 
          The capitalized costs incurred during the period (excluded from amortization) to purchase, lease or otherwise acquire an unproved property, including costs of lease bonuses and options to purchase or lease properties, the portion of costs applicable to minerals when land including mineral rights is purchased in fee, brokers' fees, recording fees, legal costs, and other costs incurred in acquiring properties. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition 
          The capitalized costs incurred (excluded from amortization), as of the date of the balance sheet, to purchase, lease or otherwise acquire an unproved property, including costs of lease bonuses and options to purchase or lease properties, the portion of costs applicable to minerals when land including mineral rights is purchased in fee, brokers' fees, recording fees, legal costs, and other costs incurred in acquiring properties. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition 
          The sum of the capitalized costs incurred during the period of unproved properties excluded from amortization including acquisition costs, exploration costs, development costs, and production costs. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition 
          The sum of the capitalized costs incurred, as of the balance sheet date, of unproved properties excluded from amortization including acquisition costs, exploration costs, development costs, and production costs. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Details 
          
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- Definition 
          Capitalized interest of unproved properties excluded from amortization No definition available. 
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- Definition 
          Capitalized interest of unproved properties excluded from amortization cumulative. No definition available. 
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