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Pre-Tax (Loss)/Income from Continuing Operations Before Impairment of Equity Investee and Share of Equity Investee's Share of Losses (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Income Taxes [Line Items]      
Income before tax, impairment of investment in equity investee and share of equity investees' losses $ (162.5) $ 712.7 $ 1,004.4
South Africa
      
Income Taxes [Line Items]      
Income before tax, impairment of investment in equity investee and share of equity investees' losses (348.7) (169.5) (124.9)
Ghana
      
Income Taxes [Line Items]      
Income before tax, impairment of investment in equity investee and share of equity investees' losses (96.9) 441.6 624.9
Australia
      
Income Taxes [Line Items]      
Income before tax, impairment of investment in equity investee and share of equity investees' losses 111.0 156.5 258.8
Peru
      
Income Taxes [Line Items]      
Income before tax, impairment of investment in equity investee and share of equity investees' losses 153.4 259.6 241.1
British Virgin Islands
      
Income Taxes [Line Items]      
Income before tax, impairment of investment in equity investee and share of equity investees' losses $ 18.7 $ 24.5 $ 4.5