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Property, Plant and Equipment (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Oil and natural gas proved properties
   
Property, Plant and Equipment    
Cumulative capitalized interest $ 20.7 $ 11.7
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 $ 10.7 $ 7.1