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Acquisitions and Divestitures - Drilling and Completion Costs Associated with Sale of Working Interests (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Drilling Carry Transaction Activities [Line Items]  
Total Drilling Carry $ 1,000.0
Drilling Carry Recorded 509.7
Drilling Carry Remaining 490.3
Atinum MidCon I, LLC
  
Drilling Carry Transaction Activities [Line Items]  
Total Drilling Carry 250.0
Drilling Carry Recorded 197.2
Drilling Carry Remaining 52.8
Repsol E&P USA, Inc.
  
Drilling Carry Transaction Activities [Line Items]  
Total Drilling Carry 750.0
Drilling Carry Recorded 312.5
Drilling Carry Remaining $ 437.5