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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, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Environmental And Mine Closure Obligations [Abstract] Ìý Ìý
Asset retirement obligation at beginning of period $ 203.7 [1] $ 168.3 [1]
Accretion expense 4.5 16.1 [1]
Exchange rate changes 0.3 0.1 [1]
Revision in estimated cash flows Ìý 5.9 [1]
Payments Ìý (0.7) [1]
Acquired through business combinations Ìý 14.0 [1]
Asset retirement obligation at end of period $ 208.5 $ 203.7 [1]
[1] Represents a 12-month rollforward of our asset retirement obligation at December 31, 2011.