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Discontinued Operations - Summary of Termination Of Contract (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Statement [LineItems]      
Profit (loss) from discontinued operations $ 0.0 $ 0.0 $ 13.1
Trade and other payables 594.4 $ 417.5 [1] $ 463.1
BCM Ghana Limited [Member]      
Statement [LineItems]      
Profit (loss) from discontinued operations 13.1    
Trade and other payables 40.3    
Mining Fleet [Member] | BCM Ghana Limited [Member]      
Statement [LineItems]      
Mining fleet 31.2    
Trade and other payables 31.2    
Premium On Minining Fleet [Member] | BCM Ghana Limited [Member]      
Statement [LineItems]      
Profit (loss) from discontinued operations 7.8    
Trade and other payables 7.8    
Demobilisation Costs [Member] | BCM Ghana Limited [Member]      
Statement [LineItems]      
Profit (loss) from discontinued operations 1.3    
Trade and other payables 1.3    
Early Terminations [Member] | BCM Ghana Limited [Member]      
Statement [LineItems]      
Profit (loss) from discontinued operations $ 4.0    
[1] Refer note 42.