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Loan Advanced - Contractor - Summary of Termination Of Contract (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Statement [line items]    
Trade and other payables $ 550.6 $ 466.8
BCM Ghana Limited [Member]    
Statement [line items]    
Profit (loss) from discontinued operations 1.1 13.1
Trade and other payables 39.5 40.3
Mining Fleet [Member] | BCM Ghana Limited [Member]    
Statement [line items]    
Mining fleet 29.4 31.2
Trade and other payables 29.4 31.2
Premium Payable On Minining Fleet [Member] | BCM Ghana Limited [Member]    
Statement [line items]    
Profit (loss) from discontinued operations (0.5) 7.8
Trade and other payables 7.3 7.8
Demobilisation Costs [Member] | BCM Ghana Limited [Member]    
Statement [line items]    
Profit (loss) from discontinued operations (0.2) 1.3
Trade and other payables 1.0 1.3
Early Terminations Payment [Member] | BCM Ghana Limited [Member]    
Statement [line items]    
Profit (loss) from discontinued operations 0.0 4.0
Non Mobile Fleet [Member] | BCM Ghana Limited [Member]    
Statement [line items]    
Profit (loss) from discontinued operations 1.8 0.0
Trade and other payables $ 1.8 $ 0.0