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Asset Retirement Obligations (Tables)
 3 Months Ended
Mar. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations Changes in ARO are as follows:
 
Three Months Ended March 31,
(millions)
2018
 
2017
Asset Retirement Obligations, Beginning Balance
$
875

 
$
935

Liabilities Incurred
2

 
1

Liabilities Settled
(20
)
 
(9
)
Revisions of Estimates
(11
)
 
(7
)
Reclassification to Liabilities Associated with Assets Held for Sale
(227
)
 
—

Accretion Expense (1)
9

 
12

Asset Retirement Obligations, Ending Balance
$
628

 
$
932


(1) 
Accretion expense is included in depreciation, depletion and amortization (DD&A) expense in the consolidated statements of operations.