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
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Asset Retirement Obligation [Roll Forward]    
Asset retirement obligation at beginning of period $ 172.4 $ 168.4
Accretion expense 2.6 9.5
Remediation payments (0.1) (1.0)
Revision in estimated cash flows 0.0 (4.5)
Asset retirement obligation at end of period $ 174.9 $ 172.4