Annual report pursuant to Section 13 and 15(d)

Borrowings (Senior Secured Revolving Credit Facility) (Details)

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Borrowings (Senior Secured Revolving Credit Facility) (Details) (USD $)
12 Months Ended 12 Months Ended 0 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2012
Revolving Credit Facility [Member]
Dec. 31, 2011
Revolving Credit Facility [Member]
Dec. 31, 2012
Revolving Credit Facility [Member]
Regions Bank [Member]
Oct. 31, 2012
Revolving Credit Facility [Member]
Regions Bank [Member]
Jun. 20, 2012
Revolving Credit Facility [Member]
Regions Bank [Member]
Jun. 19, 2012
Revolving Credit Facility [Member]
Regions Bank [Member]
Dec. 31, 2012
Revolving Credit Facility [Member]
Minimum [Member]
Regions Bank [Member]
Dec. 31, 2012
Revolving Credit Facility [Member]
Maximum [Member]
Regions Bank [Member]
Mar. 13, 2013
Subsequent Event [Member]
Revolving Credit Facility [Member]
Regions Bank [Member]
Dec. 28, 2012
Habanero Field (Garden Banks Block 341) [Member]
Shell Offshore Inc. [Member]
Line of Credit Facility [Line Items]                          
Maximum borrowing capacity               $ 200,000,000          
Current borrowing capacity       65,000,000 45,000,000   80,000,000   60,000,000        
Business Combination, Consideration Transferred, After Customary Purchase Price Adjustments                         39,410,000
Credit Facility 10,000,000 0   10,000,000 0                
Line of Credit Facility, Current Borrowing Capacity, Percent Increase       44.00%                  
Proceeds from Divestiture of Businesses 39,936,000 7,615,000 0                    
Interest rate at period end (as a percent)           2.72%              
Basis Spread on LIBOR (as a percent)                   2.50% 3.00%    
Unused capacity, commitment fee (as a percent)           0.50%              
Credit Facility                       25,000,000  
Line of Credit Facility, Increase, Additional Borrowings                       $ 15,000,000