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Asset Retirement Obligations (Details) (USD $)
In Thousands, unless otherwise specified
6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Dec. 31, 2012
Asset Retirement Obligation, Roll Forward Analysis      
Balance at December 31, 2012 $ 62,563    
Obligations incurred with development activities 211    
Accretion expense 2,481 1,644  
Revision in estimated cash flows 963    
Obligations discharged with disposal of properties and asset retirements (7,481)    
Balance at June 30, 2013 58,737    
Liabilities held for sale (22,370) [1]    
Less current portion (1,000)    
Long-term portion $ 35,367   $ 61,563
[1] Represents asset retirement obligations related to our assets held for sale. See Note 12, Assets Held for Sale, Divestitures and Discontinued Operations, for additional information regarding the planned sale of these properties.