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Segment Results and Assets (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
 6 Months Ended 12 Months Ended
Dec. 31, 2010
Dec. 31, 2012
Dec. 31, 2011
Jun. 30, 2010
Segment Reporting Information [Line Items]        
Corporate expenditure $ 24.5 $ 46.6 $ 37.6 $ 54.5
Environmental rehabilitation 10.9 28.2 24.9 19.3
Employee termination costs $ 35.3 $ 13.8 $ 32.8 $ 10.3