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Investment in Oil and Gas Properties (Textuals) (Details) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Reserve Quantities [Line Items]      
Sale of oil and gas properties $ 837,000 $ 14,000,000 $ 35,000,000
Receipt of payment, achievement of production metrics   14,000,000  
Unevaluated oil and gas properties 71,700,000 70,400,000  
Interest costs capitalized 7,000,000 7,000,000 7,800,000
Expected period to evaluate majority of unevaluated properties 3 years    
Unevaluated costs expected to be evaluated in 2013 28,300,000    
Mississippian Lime [Member]
     
Reserve Quantities [Line Items]      
Sale of oil and gas properties 6,100,000 14,500,000  
Woodford [Member]
     
Reserve Quantities [Line Items]      
Sale of oil and gas properties     36,000,000
Arkansas [Member]
     
Reserve Quantities [Line Items]      
Sale of oil and gas properties 9,200,000    
Evaluated Woodford Joint Development [Member]
     
Reserve Quantities [Line Items]      
Sale of oil and gas properties     35,000,000
Uncompleted Wells Equipment and Facilities [Member]
     
Reserve Quantities [Line Items]      
Unevaluated oil and gas properties 12,700,000 5,900,000  
Exploratory wells drilled, unevaluated 17 44  
2012 [Member]
     
Reserve Quantities [Line Items]      
Unevaluated oil and gas properties 24,800,000    
Percentage of unevaluated costs capitalized in period 35.00%    
2011 [Member]
     
Reserve Quantities [Line Items]      
Unevaluated oil and gas properties 26,500,000    
Percentage of unevaluated costs capitalized in period 37.00%    
2010 and prior [Member]
     
Reserve Quantities [Line Items]      
Unevaluated oil and gas properties $ 20,400,000    
Percentage of unevaluated costs capitalized in period 28.00%