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Variable Interest Entities - Royalty Trusts - Ownership Interest (Details)
Jun. 30, 2014
Dec. 31, 2013
Mississippian Trust I
   
Variable Interest Entity    
Beneficial interest owned by Company 26.90% 26.90%
Permian Trust
   
Variable Interest Entity    
Beneficial interest owned by Company 25.00% 28.50%
Mississippian Trust II
   
Variable Interest Entity    
Beneficial interest owned by Company 37.60% 37.60%