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Other disclosures - Additional information (Detail)
£ in Millions, $ in Millions
 6 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2018
GBP (£)
Dec. 31, 2017
USD ($)
Other Disclosures [Line Items]      
Capital commitments $ 2,562.0   $ 2,641.0
Contractual capital commitments 500.0   500.0
Contingent liabilities indemnities and other performance guarantees 522.0   552.0
Simandou [member]      
Other Disclosures [Line Items]      
Contractual payments 10.5    
United Kingdoms Financial Conduct Authority (FCA) [Member]      
Other Disclosures [Line Items]      
Financial penalty 36.4 £ 27.4  
Joint ventures [Member]      
Other Disclosures [Line Items]      
Capital commitments $ 153.0   $ 29.0