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Concentrations (Tables)
12 Months Ended
Dec. 31, 2011
Concentrations [Abstract]  
Sales to Major Customers
                         
    December 31,  
    2011     2010     2009  

Conoco, Inc.

    28     26     27

Hess Corporation

    (a     11     11

Sequent Energy Management LP

    (a     10     13

Shell Trading (US) Company

    46     40     34
       

(a) Less than 10 percent