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Estimated Capital Allowances (Parenthetical) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Operating Loss Carryforwards [Line Items]    
Calculated tax losses $ 652.3 $ 739.1
Abosso Goldfields Limited    
Operating Loss Carryforwards [Line Items]    
Maximum number of years losses can be carried forward 5 years  
Calculated tax losses [1] $ 65.7 46.5
Exploration Entities    
Operating Loss Carryforwards [Line Items]    
Calculated tax losses [2] 345.2 387.9
Tax losses without expiration 188.2 224.6
Expire Between One and Two Years | Exploration Entities    
Operating Loss Carryforwards [Line Items]    
Tax losses with expiration 3.8 12.1
Expire Between Two and Five Years | Exploration Entities    
Operating Loss Carryforwards [Line Items]    
Tax losses with expiration 62.9 67.1
Expire Between Five and Ten Years | Exploration Entities    
Operating Loss Carryforwards [Line Items]    
Tax losses with expiration 49.6 62.5
Expire After 10 Years | Exploration Entities    
Operating Loss Carryforwards [Line Items]    
Tax losses with expiration $ 40.7 $ 21.6
[1] Tax losses may be carried forward for five years. These losses expire on a first-in-first-out basis.
[2] The tax losses of $345.2 million (2014: $387.9 million) comprise $3.8 million (2014: $12.1 million) of tax losses that expire between one and two years, $62.9 million (2014: $67.1 million) of tax losses that expire between two and five years, $49.6 million (2014: $62.5 million) of tax losses that expire between five and ten years, $40.7 million (2014: $21.6 million) of tax losses that expire after 10 years and $188.2 million (2014: $224.6 million) of tax losses that have no expiry date.