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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, 2011
Jun. 30, 2010
Jun. 30, 2009
Income Taxes [Line Items]        
Income before tax, impairment of investment in equity investee and share of equity investees' losses $ 194.8 $ 1,507.8 $ 851.4 $ 551.2
SOUTH AFRICA
        
Income Taxes [Line Items]        
Income before tax, impairment of investment in equity investee and share of equity investees' losses (221.0) 378.5 192.0 269.9
GHANA
        
Income Taxes [Line Items]        
Income before tax, impairment of investment in equity investee and share of equity investees' losses 216.8 624.9 343.1 140.7
AUSTRALIA
        
Income Taxes [Line Items]        
Income before tax, impairment of investment in equity investee and share of equity investees' losses 77.3 258.8 55.7 92.3
PERU
        
Income Taxes [Line Items]        
Income before tax, impairment of investment in equity investee and share of equity investees' losses 139.4 241.1 164.6 82.7
British Virgin Islands
        
Income Taxes [Line Items]        
Income before tax, impairment of investment in equity investee and share of equity investees' losses $ (17.7) [1] $ 4.5 [1] $ 96.0 [1] $ (34.4) [1]
[1] The pre-tax loss relates to non-operating entitites, incorporated in the British Virgin Islands, includes impairment of investments for fiscal year ended December 31, 2011; net gain on disposal of investments in the six-month period ended December 31, 2010, a net gain on disposal of investments and impairment of investments in fiscal year ended June 30, 2010 and a net loss on disposal and impairment of listed investments in fiscal year ended June 30, 2009.