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Asset Retirement Obligations (Tables)
12 Months Ended
Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Reconciliation of Changes in Asset Retirement Obligations [Table Text Block]
The following is a reconciliation of the changes in the ARO from January 1, 2017 to December 31, 2018:
(In millions)
2018
 
2017
AROs at beginning of period
$
28

 
$
25

Liabilities incurred
1

 
2

Accretion expense
1

 
1

AROs at end of period
$
30

 
$
28