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Asset Retirement Obligations (Tables)
 6 Months Ended
Jun. 30, 2017
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations
Changes in ARO are as follows:
 
Six Months Ended June 30,
(millions)
2017
 
2016
Asset Retirement Obligations, Beginning Balance
$
935

 
$
989

Liabilities Incurred
82

 
3

Liabilities Settled
(32
)
 
(38
)
Revision of Estimate
(15
)
 
4

Accretion Expense (1)
23

 
25

Asset Retirement Obligations, Ending Balance
$
993

 
$
983


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