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Asset Retirement Obligations (Tables)
 3 Months Ended
Mar. 31, 2014
Asset Retirement Obligation [Abstract]  
Asset retirement obligations roll forward

 
Asset Retirement Obligations
 
2014
 
(in millions)
ARO liability at January 1,
$
193.6

Accretion
2.0

Additions(1)
10.9

Revisions
(0.3
)
Liabilities settled
(0.8
)
ARO liability at March 31,
$
205.4