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Equity share capital - Summary of Cash Flow Hedging Reserve (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of detailed information about hedged items [line items]      
Costs of hedging deferred and recognised in OCI $ (1)    
Reclassified from OCI to profit or loss - included in financial expenses (8)    
Deferred tax (1) $ 0 $ 0
Reserve Of Cash flow Hedges Ending Balance (4)    
Cash flow hedging reserve [member]      
Disclosure of detailed information about hedged items [line items]      
Reserve Of Cash flow Hedges Beginning Balance 0    
Costs of hedging deferred and recognised in OCI (1)    
Change in fair value of currency swaps recognised in OCI 4    
Reclassified from OCI to profit or loss - included in financial expenses (8)    
Deferred tax 1    
Reserve Of Cash flow Hedges Ending Balance (4) 0  
Cash flow hedging reserve [member] | Value of currency swaps [member]      
Disclosure of detailed information about hedged items [line items]      
Reserve Of Cash flow Hedges Beginning Balance 0    
Costs of hedging deferred and recognised in OCI 0    
Change in fair value of currency swaps recognised in OCI 4    
Reclassified from OCI to profit or loss - included in financial expenses (8)    
Deferred tax 1    
Reserve Of Cash flow Hedges Ending Balance (3) 0  
Cash flow hedging reserve [member] | Costs of hedging [member]      
Disclosure of detailed information about hedged items [line items]      
Reserve Of Cash flow Hedges Beginning Balance 0    
Costs of hedging deferred and recognised in OCI (1)    
Change in fair value of currency swaps recognised in OCI 0    
Reclassified from OCI to profit or loss - included in financial expenses 0    
Deferred tax 0    
Reserve Of Cash flow Hedges Ending Balance $ (1) $ 0