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Asset Retirement Obligations (Tables)
 6 Months Ended
Jun. 30, 2012
Asset Retirement Obligations [Abstract]  
Asset retirement obligations roll forward
The following is a reconciliation of the changes in the asset retirement obligation from January 1, 2012 and 2011 to June 30, 2012 and 2011, respectively:
 
 
Asset Retirement Obligations
 
 
2012
 
 
2011
 
 
(in millions)
 
ARO liability at January 1,
 
$
163.9
 
 
$
148.3
 
Accretion
 
 
5.1
 
 
 
4.8
 
Liabilities incurred
 
 
3.6
 
 
 
2.9
 
Liabilities settled
 
 
(0.4
)
 
 
(0.3
)
ARO liability at June 30,
 
$
172.2
 
 
$
155.7