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Business Segment Information - Major Customers (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Revenue, Major Customer [Line Items]                      
Sales $ 465,108 $ 493,603 $ 512,987 $ 511,690 $ 1,338,098 $ 532,798 $ 478,434 $ 381,635 $ 1,983,388 $ 2,730,965 $ 1,415,213
Occidental Petroleum Corporation
                     
Revenue, Major Customer [Line Items]                      
Sales                   829,081  
Percentage of revenue                   30.40%  
Plains Marketing, L.P.
                     
Revenue, Major Customer [Line Items]                      
Sales                 491,258 426,339 276,285
Percentage of revenue                 24.80% 15.60% 19.50%
Enterprise Crude Oil, LLC
                     
Revenue, Major Customer [Line Items]                      
Sales                   394,162 319,277
Percentage of revenue                   14.40% 22.60%
Shell Trading (US) Company
                     
Revenue, Major Customer [Line Items]                      
Sales                 347,422    
Percentage of revenue                 17.50%    
Atlas Pipeline Mid-Continent West OK LLC
                     
Revenue, Major Customer [Line Items]                      
Sales                 $ 211,838    
Percentage of revenue                 10.70%