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Asset Retirement Obligations (Tables)
 6 Months Ended
Jun. 30, 2015
Asset Retirement Obligation [Abstract]  
Asset retirement obligations roll forward
 
Asset Retirement Obligations
 
2015
 
(in millions)
ARO liability at January 1,
$
195.1

Accretion
4.3

Additions
1.9

Revisions
0.2

Liabilities related to divestitures
(14.6
)
Liabilities settled
(1.0
)
ARO liability at June 30,
$
185.9