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Asset Retirement Obligations
 12 Months Ended
Dec. 31, 2019
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations
Asset Retirement Obligations
The components of the change in our asset retirement obligations are shown below:
 
 
Years Ended December 31,
 
 
2019
 
2018
 
 
($ in millions)
Asset retirement obligations, beginning of period
 
$
166

 
$
177

Additions(a)
 
21

 
3

Revisions
 
18

 
11

Settlements and disposals
 
(5
)
 
(35
)
Accretion expense
 
11

 
10

Asset retirement obligations, end of period
 
211

 
166

Less current portion
 
11

 
11

Asset retirement obligation, long-term
 
$
200

 
$
155


___________________________________________
(a)
During 2019, approximately $17 million of additions relate to the acquisition of WildHorse.