Quarterly report pursuant to Section 13 or 15(d)

Environmental And Mine Closure Obligations (Asset Retirement Obligation Disclosure) (Detail)

v2.4.0.6
Environmental And Mine Closure Obligations (Asset Retirement Obligation Disclosure) (Detail) (USD $)
In Millions, unless otherwise specified
9 Months Ended 12 Months Ended
Sep. 30, 2012
Dec. 31, 2011
Asset Retirement Obligation [Roll Forward]    
Asset retirement obligation at beginning of period $ 194.9 $ 162.1
Accretion expense 13.2 15.7
Exchange rate changes 0.2 0.1
Revision in estimated cash flows    3.7
Payments    (0.7)
Acquired through business combinations    14.0
Asset retirement obligation at end of period $ 208.3 $ 194.9