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Commitments - Schedule of Commitments (Detail) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Capital expenditure    
Contracted for Operating leases $ 47.9 $ 50.0
Within one year [member]    
Capital expenditure    
Operating lease commitment 0.0 76.7
Later than one and not later than five years [member]    
Capital expenditure    
Operating lease commitment 0.0 256.5
Later than five years [member]    
Capital expenditure    
Operating lease commitment $ 0.0 $ 324.2