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Asset Retirement Obligations (Tables)
 9 Months Ended
Sep. 30, 2015
Asset Retirement Obligation [Abstract]  
Schedule of asset retirement obligations
The following table summarizes the Company's asset retirement obligation activity during the three and nine months ended September 30, 2015 and 2014: 
 
 
Three Months Ended
September 30,
 
Nine Months Ended
September 30,
 
 
2015
 
2014
 
2015
 
2014
 
 
(in millions)
Beginning asset retirement obligations
 
$
187

 
$
193

 
$
189

 
$
194

New wells placed on production
 
1

 
1

 
2

 
3

Changes in estimates
 
—

 
2

 
—

 
3

Dispositions
 
—

 
(5
)
 
—

 
(7
)
Liabilities settled
 
(9
)
 
(6
)
 
(18
)
 
(14
)
Accretion of discount
 
3

 
3

 
9

 
9

Ending asset retirement obligations
 
$
182

 
$
188

 
$
182

 
$
188