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Royalties - Summary of Royalties (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of information about royalty arrangements [line items]      
Total royalties $ (62.5) $ (62.0) $ (78.4)
South Africa [member]      
Disclosure of information about royalty arrangements [line items]      
Total royalties $ (1.0) $ (1.8) $ (1.8)
Royalty percentage 0.50% 0.50% 0.50%
Foreign [member]      
Disclosure of information about royalty arrangements [line items]      
Total royalties $ (61.5) $ (60.2) $ (76.6)
Australia [member]      
Disclosure of information about royalty arrangements [line items]      
Total royalties $ (27.9) $ (27.8) $ (27.3)
Royalty percentage 2.50% 2.50% 2.50%
Ghana [member]      
Disclosure of information about royalty arrangements [line items]      
Total royalties $ (31.3) $ (27.1) $ (44.6)
Royalty percentage 3.00% 3.00% 5.00%
Peru [member]      
Disclosure of information about royalty arrangements [line items]      
Royalty percentage 4.00% 4.60% 6.40%