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Pre-Tax Income Before Impairment of Equity Investee and Share of Equity Investee's Share of Losses (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
Income Taxes [Line Items]        
Income before tax, impairment of investment in equity investee and share of equity investees' losses $ 194.8 $ 996.1 $ 1,507.8 $ 851.4
South Africa
        
Income Taxes [Line Items]        
Income before tax, impairment of investment in equity investee and share of equity investees' losses (221.0) 113.9 378.5 192.0
Ghana
        
Income Taxes [Line Items]        
Income before tax, impairment of investment in equity investee and share of equity investees' losses 216.8 441.6 624.9 343.1
Australia
        
Income Taxes [Line Items]        
Income before tax, impairment of investment in equity investee and share of equity investees' losses 77.3 156.5 258.8 55.7
Peru
        
Income Taxes [Line Items]        
Income before tax, impairment of investment in equity investee and share of equity investees' losses 139.4 259.6 241.1 164.6
British Virgin Islands
        
Income Taxes [Line Items]        
Income before tax, impairment of investment in equity investee and share of equity investees' losses $ (17.7) $ 24.5 $ 4.5 $ 96.0