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Asset Retirement Obligations (Tables)
 6 Months Ended
Jun. 30, 2014
Asset Retirement Obligation Disclosure [Abstract]  
Changes in Asset Retirement Obligations
Changes in ARO are as follows:
 
Six Months Ended
June 30,
(millions)
2014
 
2013
Asset Retirement Obligations, Beginning Balance
$
586

 
$
402

Liabilities Incurred
22

 
2

Liabilities Settled
(43
)
 
(10
)
Revision of Estimate
120

 
7

Accretion Expense (1)
19

 
14

Asset Retirement Obligations, Ending Balance
$
704

 
$
415


(1) Accretion expense is included in DD&A expense in the consolidated statements of operations.