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, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Asset Retirement Obligation [Roll Forward]    
Asset retirement obligation at beginning of period $ 231.1 $ 194.9
Accretion expense 4.5 17.6
Exchange rate changes    0.3
Revision in estimated cash flows (17.2) 18.2
Payments (0.1) 0.1
Asset retirement obligation at end of period $ 218.3 $ 231.1