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Decommissioning Liabilities (Tables)
 12 Months Ended
Dec. 31, 2017
Asset Retirement Obligation Disclosure1 [Abstract]  
Schedule of Change in Asset Retirement Obligation
A continuity of the decommissioning liability is as follows:
 
Total
Balance – December 31, 2015
$37.2

Reclamation expenditures
(0.3
)
Accretion expense
2.1

Revisions to expected discounted cash flows
0.6

Balance – December 31, 2016
$39.6
Liability assumed on acquisition
2.4

Reclamation expenditures
(0.1
)
Accretion expense
2.6

Revisions to expected discounted cash flows
0.1

Balance – December 31, 2017
$44.6