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Asset Retirement Obligations Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of Asset Retirement Obligations
The components of the change in our asset retirement obligations are shown below:
 
 
Years Ended December 31,
 
 
2017
 
2016
 
 
($ in millions)
Asset retirement obligations, beginning of period
 
$
261

 
$
473

Additions
 
5

 
4

Revisions
 
(34
)
 
(58
)
Settlements and disposals
 
(70
)
 
(182
)
Accretion expense
 
15

 
24

Asset retirement obligations, end of period
 
177

 
261

Less current portion
 
15

 
14

Asset retirement obligation, long-term
 
$
162

 
$
247