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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2016
Asset Retirement Obligation [Abstract]  
Changes to Asset Retirement Obligation
The following table describes changes to the asset retirement obligation liability (dollars in millions):
 
Year Ended December 31
 
2016
 
2015
Asset retirement obligation at beginning of period
$
26.2

 
$
37.0

Accretion expense
2.3

 
2.1

Payments
(2.2
)
 
(1.6
)
Revisions in estimated cash flows
1.2

 
0.2

Liabilities incurred
0.5

 
(0.3
)
Sale of St. Helens (a)
—

 
(11.2
)
Asset retirement obligation at end of period
$
28.0

 
$
26.2