Quarterly report pursuant to Section 13 or 15(d)

ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Asset Retirement Obligation Disclosure) (Details)

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ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Asset Retirement Obligation Disclosure) (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Asset Retirement Obligation [Roll Forward]    
Asset retirement obligation at beginning of period $ 168.4 $ 187.8
Accretion expense 4.7 13.9
Remediation payments (0.3) (5.6)
Revision in estimated cash flows 0.0 (27.7)
Asset retirement obligation at end of period $ 172.8 $ 168.4