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Asset Retirement Obligations (Tables)
 6 Months Ended
Jun. 30, 2017
Asset Retirement Obligation [Abstract]  
Schedule of asset retirement obligations
The following table summarizes the Company's asset retirement obligation activity during the three and six months ended June 30, 2017 and 2016: 
 
 
Three Months Ended
June 30,
 
Six Months Ended
June 30,
 
 
2017
 
2016
 
2017
 
2016
 
 
(in millions)
Beginning asset retirement obligations
 
$
297

 
$
285

 
$
297

 
$
285

New wells placed on production
 
1

 
—

 
1

 
—

Changes in estimates
 
7

 
—

 
7

 
—

Dispositions
 
(7
)
 
—

 
(7
)
 
—

Liabilities settled
 
(9
)
 
(9
)
 
(14
)
 
(13
)
Accretion of discount
 
5

 
5

 
10

 
9

Ending asset retirement obligations
 
$
294

 
$
281

 
$
294

 
$
281