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Derivatives (Narrative) (Details) (USD $)
 6 Months Ended 0 Months Ended 6 Months Ended 7 Months Ended 17 Months Ended 0 Months Ended 17 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jul. 28, 2009
Costless Collar [Member]
Jun. 30, 2012
Costless Collar [Member]
Jul. 31, 2011
Costless Collar [Member]
bbl
Dec. 31, 2010
Costless Collar [Member]
bbl
Jun. 27, 2011
Cargill Agreement [Member]
Dec. 31, 2012
Cargill Agreement [Member]
bbl
Derivative [Line Items]                
Price agreement term     two       If the average price falls below $65 per barrel, then Cargill will pay to the Company the difference between $65 and the lower average price for 10,000 barrels per month in each month during when such lower average prices occur.  
Floor price (per barrel)     60.00       65  
Cap price (per barrel)     81.50          
Monthly volume (in barrels)         7,375 9,500   10,000
Difference per barrel of Kansas Common plus bonus and current WTI NYMEX       7        
Contract description     two       If the average price falls below $65 per barrel, then Cargill will pay to the Company the difference between $65 and the lower average price for 10,000 barrels per month in each month during when such lower average prices occur.  
Derivative issuance cost per barrel             2.20  
Payments of derivative issuance costs             $ 374,000  
Unrealized gain (loss) on derivatives (105,000) 439,000            
Settlement payments   $ (745,000)