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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2018
Asset Retirement Obligation [Abstract]  
Changes to the asset retirement obligations
The following table summarizes the changes to the ARO balances during 2018 and 2017:
ARO Reconciliation
 
(In millions)
 
 
 
Balance, January 1, 2017
 
$
581

Transfer of BV-2 liability to NG
 
(49
)
Liabilities settled
 
(1
)
Accretion
 
39

Balance, December 31, 2017
 
$
570

Changes in timing and amount of estimated cash flows
 
203

Liabilities settled
 
(1
)
Accretion
 
40

Balance, December 31, 2018
 
$
812