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Property, Plant and Equipment (Parenthetical) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Oil and gas proved properties
   
Property, Plant and Equipment [Line Items]    
Cumulative capitalized interest $ 11.7 $ 1.6
Non-oil and natural gas equipment
   
Property, Plant and Equipment [Line Items]    
Cumulative capitalized interest $ 11.4 $ 6.7