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Commitments - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Guarantees and Other Commitments [Line Items]      
Operating lease charges $ 2.7 $ 3.2 $ 4.5
Environmental Guarantee Obligations      
Guarantees and Other Commitments [Line Items]      
Guarantees 0.0 0.1  
Environmental Guarantee Obligations | South African and Ghanaian Operations [Member]      
Guarantees and Other Commitments [Line Items]      
Guarantees $ 59.0 $ 67.0