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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, 2018
Dec. 31, 2017
Asset Retirement Obligation [Roll Forward] Ìý Ìý
Asset retirement obligation at beginning of period $ 197.2 $ 204.0
Accretion expense 2.7 14.9
Asset Retirement Obligation, Liabilities Settled (0.1) (5.6)
Exchange rate changes (0.5) 1.5
Revision in estimated cash flows 0.1 (17.6)
Asset retirement obligation at end of period $ 199.4 $ 197.2