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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2013
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of changes in asset retirement obligations
Changes in asset retirement obligations for the years ended December 31, 2013 and 2012 were as follows:
(in millions)
 
2013
 
2012
Balance at beginning of year
 
$
13.0

 
$
12.8

Accretion expense
 
1.1

 
1.1

Payments
 
(0.8
)
 
(0.6
)
Revision of estimates
 
13.8

 
(0.3
)
Liabilities incurred
 
0.6

 
—

Balance at end of year
 
$
27.7

 
$
13.0