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Asset Retirement Obligations ('ARO')
 12 Months Ended
Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations ('ARO')
Asset Retirement Obligations
The reconciliation of the changes in our AROs is presented below:
 
Year ended December 31,
 
2018
 
2017
Beginning balance, ARO liability
$
28.5

 
$
35.4

Additional ARO liability incurred
3.6

 
1.4

ARO liabilities settled
(6.5
)
 
(9.9
)
Accretion expense
1.9

 
1.9

Revisions in estimated cash flows
(9.3
)
 
(0.3
)
Ending balance, ARO liability
$
18.2

 
$
28.5


Revisions in estimated cash flows during 2018 were related to a change in estimate of future disposal costs of spent potliners.