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Commitments - Additional Information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of Information about Guarantees [Line Items]      
Leases Use in Assets Not Yet Commenced $ 33.4    
South African Peruvian and Ghanaian Operations [member]      
Disclosure of Information about Guarantees [Line Items]      
Environmental obligation guarantees amount $ 154.3 $ 207.6 $ 112.1