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, 2016
Dec. 31, 2015
Asset Retirement Obligation [Roll Forward]    
Asset retirement obligation at beginning of period $ 206.3 $ 142.4
Accretion expense 2.5 6.5
Exchange rate changes 0.9 (1.1)
Revision in estimated cash flows (20.8) 58.5
Asset retirement obligation at end of period $ 188.9 $ 206.3