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Asset Impairment (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
Impairment Of Assets [Line Items]        
Asset impairment charge    $ 41.6 $ 9.5   
Heap leach write-down market value
        
Impairment Of Assets [Line Items]        
Asset write-down charge   19.2 [1]    
Property Plant and Equipment
        
Impairment Of Assets [Line Items]        
Asset impairment charge   14.5 [2] 9.5 [2]  
Australia | Heap leach write-down market value
        
Impairment Of Assets [Line Items]        
Asset write-down charge   19.2    
Australia | Property Plant and Equipment | Heap leach Asset
        
Impairment Of Assets [Line Items]        
Asset impairment charge   10.1    
Ghana | Property Plant and Equipment
        
Impairment Of Assets [Line Items]        
Asset impairment charge   $ 4.4    
[1] The charge of $19.2 million relates to a write-down to market value in Australia due to the cessation of the heap leach at St. Ives.
[2] The impairment charge in fiscal 2012 consisted of a $10.1 million impairment of heap leach assets in Australia due to the cessation of the heap leach at St. Ives and $4.4 million impairment of heavy mining machinery in Ghana. The impairment charge in fiscal 2011 resulted from the decision to reassess the optimal processing methodology for the oxides at Cerro Corona, where the focus was on the evaluation of a heap leach operation to capture the value inherent in the oxide instead of a stand-alone oxide plant; the evaluation costs of which were written off in 2011.