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Asset Retirement Obligations (Tables)
12 Months Ended
Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of company's ARO obligations and related additions, reductions and accretion
The following table represents the balance of ARO obligations as of December 31, 2017 and 2016, along with the additions, reductions and accretion related to the Company's ARO obligations for the year ended December 31, 2017:
 
(In millions)
Balance as of December 31, 2016
$
735

Revisions in estimates for current obligations
(3
)
Additions
9

Spending for current obligations
(21
)
Accretion — Expense
35

Accretion — Nuclear decommissioning
16

Balance as of December 31, 2017
$
771