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Annual report pursuant to Section 13 and 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
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Asset Retirement Obligation [Roll Forward] Ìý Ìý
Asset Retirement Obligation at Beginning of Year $ 172.4 $ 168.4
Asset Retirement Obligation, Accretion Expense 10.1 9.5
Asset Retirement Obligation, Liabilities Settled (0.8) (1.0)
Asset Retirement Obligation, Revision of Estimate (16.4) (4.5)
Asset Retirement Obligation at End of Year $ 165.3 $ 172.4