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Asset Retirement Obligations
 12 Months Ended
Dec. 31, 2012
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligation
Asset Retirement Obligations

The following table presents the balance and activity of the asset retirement obligations for the years ended December 31 (in thousands).
 
2012
 
2011
 
2010
Asset retirement obligations at January 1
$
128,116

 
$
119,877

 
$
111,137

Liability incurred upon acquiring and drilling wells
7,479

 
5,716

 
17,347

Liability assumed in acquisition
371,365

 
—

 
—

Revisions in estimated cash flows
34,654

 
7,574

 
(17,017
)
Liability settled or disposed in current period
(72,200
)
 
(14,419
)
 
(1,011
)
Accretion
28,996

 
9,368

 
9,421

Asset retirement obligations at December 31
498,410

 
128,116

 
119,877

Less: current portion
118,504

 
32,906

 
25,360

Asset retirement obligations, net of current
$
379,906

 
$
95,210

 
$
94,517



    
Liability assumed in acquisition, liability settled and accretion for the year ended December 31, 2012 include amounts attributable to asset retirement obligations assumed in the acquisitions of oil and natural gas properties in the Gulf of Mexico during the second quarter of 2012. The current year settlements increased significantly due to the higher costs associated with plugging and abandoning the properties in the Gulf of Mexico. Liability settled or disposed for the year ended December 31, 2011 primarily consists of amounts related to the Permian Basin and east Texas properties sold during 2011. The revisions in estimated cash flows for the year ended December 31, 2010 were primarily due to lengthening reserve lives based on higher oil and natural gas prices used to determine reserves relative to prices at the beginning of 2010. At December 31, 2010, asset retirement obligations of $21.8 million related to an offshore platform were moved to current, due to its then anticipated plugging and abandonment in 2011.