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Asset Retirement Obligations (Tables)
 6 Months Ended
Jun. 30, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations Changes in ARO are as follows:
 
Six Months Ended June 30,
(millions)
2018
 
2017
Asset Retirement Obligations, Beginning Balance
$
875

 
$
935

Liabilities Incurred
14

 
82

Liabilities Settled
(261
)
 
(32
)
Revisions of Estimates
(10
)
 
(15
)
Accretion Expense (1)
17

 
23

Asset Retirement Obligations, Ending Balance
$
635

 
$
993


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