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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]  
Summary of Changes in Asset Retirement Obligations
The schedule below summarizes the changes in our asset retirement obligations (in millions):
 
Year Ended December 31,
 
2017
 
2016
Balance, beginning of period
$
13.5

 
$
14.0

Revisions to the fair values of existing liabilities
—

 
(0.5
)
Accretion expense
0.7

 
0.6

Liabilities settled
—

 
(0.6
)
Balance, end of period
$
14.2

 
$
13.5