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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2016
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of Change in Asset Retirement Obligation
Changes in asset retirement obligations were as follows:
 
Year Ended December 31,
(millions)
2016
 
2015
Asset Retirement Obligations, Beginning Balance
$
989

 
$
751

Liabilities Incurred
21

 
67

Liabilities Settled
(120
)
 
(38
)
Revision of Estimate
(3
)
 
166

Accretion Expense
48

 
43

Asset Retirement Obligations, Ending Balance
$
935

 
$
989