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Asset Retirement Obligations (Tables)
3 Months Ended
Mar. 31, 2015
Asset Retirement Obligation [Abstract]  
Asset retirement obligations roll forward
 
Asset Retirement Obligations
 
2015
 
(in millions)
ARO liability at January 1,
$
195.1

Accretion
2.1

Additions
0.6

Liabilities settled
(0.6
)
ARO liability at March 31,
$
197.2