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Property, Plant and Equipment (Parenthetical) (Details) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2014
Dec. 31, 2013
Oil and natural gas proved properties
   
Property, Plant and Equipment    
Cumulative capitalized interest $ 29.0 $ 23.4
Non-oil and natural gas equipment
   
Property, Plant and Equipment    
Cumulative capitalized interest 4.3 4.3
Buildings and structures
   
Property, Plant and Equipment    
Cumulative capitalized interest $ 14.4 $ 12.0