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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2020
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of Reconciliation of the Carrying Amount of AROs

A reconciliation of the beginning and ending aggregate carrying amount of AROs is as follows:

 

(millions)

 

2020

 

 

2019

 

Beginning balance

 

$

489

 

 

$

541

 

Liabilities settled

 

 

(4

)

 

 

(29

)

Accretion expense

 

 

23

 

 

 

23

 

Revisions in estimated cash flows(1)

 

 

89

 

 

 

(46

)

Ending balance

 

$

597

 

 

$

489

 

(1)   The increase in 2020 reflects revisions from the nuclear decommissioning cost study. The decrease in 2019 reflects a change in the estimated timing of cash flows for interim pipeline replacements and DOE recoveries.