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Asset Retirement Obligations (Tables)
 3 Months Ended
Mar. 31, 2019
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations Changes in ARO are as follows:
 
Three Months Ended March 31,
(millions)
2019
 
2018
Asset Retirement Obligations, Beginning Balance
$
880

 
$
875

Liabilities Incurred
2

 
2

Liabilities Settled
(27
)
 
(20
)
Revisions of Estimates
—

 
(11
)
Reclassification to Liabilities Associated with Assets Held for Sale
—

 
(227
)
Accretion Expense
12

 
9

Asset Retirement Obligations, Ending Balance
$
867

 
$
628