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Mining and Income Taxation - Summary of Domestic and Foreign Current Tax Rates (Detail)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Domestic and Foreign Current Tax Rate [Line Items]      
Current tax rates 30.00% 30.00%  
South African - Components of Mining and Income Tax [Member] | South African Mining Taxation [Member]      
Disclosure of Domestic and Foreign Current Tax Rate [Line Items]      
South African Mining Tax Formula South African mining tax on mining income is determined according to a formula Y=34-170/X in 2016, 2015 and 2014 South African mining tax on mining income is determined according to a formula Y=34-170/X in 2016, 2015 and 2014 South African mining tax on mining income is determined according to a formula Y=34-170/X in 2016, 2015 and 2014
South African - Components of Mining and Income Tax [Member] | Non Mining Tax [Member]      
Disclosure of Domestic and Foreign Current Tax Rate [Line Items]      
Current tax rates 28.00% 28.00% 28.00%
South African - Components of Mining and Income Tax [Member] | South Africa Tax [Member]      
Disclosure of Domestic and Foreign Current Tax Rate [Line Items]      
Current tax rates 28.00% 28.00% 28.00%
Australia Taxation [Member] | Australia Tax [Member]      
Disclosure of Domestic and Foreign Current Tax Rate [Line Items]      
Current tax rates 30.00% 30.00% 30.00%
Ghana Taxation [Member] | Ghana Tax [Member]      
Disclosure of Domestic and Foreign Current Tax Rate [Line Items]      
Current tax rates 32.50% 32.50% 35.00%
Peru Taxation [Member] | Peru Tax [Member]      
Disclosure of Domestic and Foreign Current Tax Rate [Line Items]      
Current tax rates 29.50% 30.00% 30.00%