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Financial instruments and risk management - Summary of changes in fair value of Level 3 financial assets and financial liabilities (Detail) - Level 3 [member] - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Financial Instruments [Line Items]    
Opening balance $ (7) $ 479
Adjustment from transition to IFRS 9 19  
Currency translation adjustments (23) 8
– consolidated sales revenue   1
– net operating costs 9 (5)
- consolidated sales revenue   17
– net operating costs 375 (508)
Total unrealised gains transferred into other comprehensive income through cash flow hedges 181  
Additions 67  
Disposals/maturity of financial instruments (6) (5)
Transfers 22 6
Closing balance 637 (7)
Total gains/(losses) for the year included in the income statement for assets and liabilities held at year end $ 346 $ (491)