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Acquisitions and Divestitures (Riley Ridge Phase 2 PPA) (Details 2) (USD $)
Dec. 31, 2010
Dec. 31, 2011
Riley Ridge Phase 2 [Member]
Dec. 31, 2011
Proved Reserves [Member]
Riley Ridge Phase 2 [Member]
Dec. 31, 2011
Unevaluated Reserves [Domain]
Riley Ridge Phase 2 [Member]
Business Combination, Consideration Transferred [Abstract]        
Cash payment $ 833,900,000 $ 199,779,000    
Deferred payment   15,000,000    
Total consideration   214,779,000    
Fair value of assets acquired and liabilities assumed:        
Business Acquisition, Purchase Price Allocation, Natural Resources     48,731,000 12,542,000
CO2 properties   9,741,000    
Pipelines and plants   91,594,000    
Other assets   48,660,000    
Asset retirement obligations   (389,000)    
Business Acquisition Purchase Price Allocation Assets Acquired Liabilities Assumed Excluding Contingent Consideration   210,879,000    
Goodwill   3,900,000    
Helium extraction rights   $ 36,700,000