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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2014
Asset Retirement Obligation [Abstract]  
Asset retirement obligations roll forward
The following is a reconciliation of the changes in the Company's ARO for the periods specified below:
 
Asset Retirement Obligations
 
2014
 
2013
 
(in millions)
ARO liability at January 1,(1)
$
165.1

 
$
155.6

Accretion
6.7

 
5.6

Additions(2)
17.1

 
6.9

Revisions
33.6

 
11.8

Liabilities related to assets sold
(24.7
)
 
(11.8
)
Liabilities settled
(2.7
)
 
(3.0
)
ARO liability at December 31,
$
195.1

 
$
165.1