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Asset Retirement Obligations (Tables)
 3 Months Ended
Mar. 31, 2018
Asset Retirement Obligation [Abstract]  
Schedule of asset retirement obligations
The following table summarizes the Company's asset retirement obligation activity during the three months ended March 31, 2018 and 2017: 
 
Three Months Ended
March 31,
 
2018
 
2017
 
(in millions)
Beginning asset retirement obligations
$
271

 
$
297

New wells placed on production
1

 
1

Changes in estimates
2

 
—

Obligations reclassified to liabilities held for sale
(6
)
 
—

Liabilities settled
(9
)
 
(6
)
Accretion of discount
4

 
5

Ending asset retirement obligations
$
263

 
$
297