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Undiscounted Closure Cost Estimates (Parenthetical) (Detail) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Asset Retirement Obligations [Line Items]      
Reconciliation of closure cost estimate, discount $ 77.5 $ 90.7  
Total closure cost estimate [1] 353.2 390.8  
Provision for environmental rehabilitation $ 275.7 $ 300.1 $ 269.2
[1] Includes discounting of $77.5 million in the year ended December 31, 2015 (December 31, 2014: $90.7 million) in order to reconcile the gross closure cost estimate of $353.2 million (December 31, 2014: $390.8 million) to the provision for environmental rehabilitation of $275.7 million (December 31, 2014: $300.1 million)