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Commitments - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2011
Dec. 31, 2010
Jun. 30, 2010
Commitments and Contingencies Disclosure [Line Items]      
Guarantees $ 1.7 $ 2.2 $ 2.6
Department of Minerals and Energy in South Africa
      
Commitments and Contingencies Disclosure [Line Items]      
Guarantees 1.2    
Guarantee Type, Other
      
Commitments and Contingencies Disclosure [Line Items]      
Guarantees 0.5    
Ghanaian and Australian Asset Retirement Obligation
      
Commitments and Contingencies Disclosure [Line Items]      
Guarantees 56.3 49.3 45.0
Total Guarantees
      
Commitments and Contingencies Disclosure [Line Items]      
Guarantees $ 1.7