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Post-retirement benefits - Summary of movement in net defined benefit liability (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance $ (1,551) $ (2,499)  
Amounts recognised in Income statement (146) (288) $ (245)
Amounts recognised in Other comprehensive income (262) 907 6
Contributions by employer 257 248 404
Arrangements divested (5) (10)  
Assets transferred to defined contribution section (3) 0  
Currency exchange rate (loss)/gain (20) 91  
Ending balance (1,730) (1,551) $ (2,499)
Pension benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (680)    
Amounts recognised in Income statement (104)    
Amounts recognised in Other comprehensive income (259)    
Contributions by employer 223    
Arrangements divested (5)    
Assets transferred to defined contribution section (3)    
Currency exchange rate (loss)/gain (3)    
Ending balance (831) (680)  
Other Benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (871)    
Amounts recognised in Income statement (42)    
Amounts recognised in Other comprehensive income (3)    
Contributions by employer 34    
Arrangements divested 0    
Assets transferred to defined contribution section 0    
Currency exchange rate (loss)/gain (17)    
Ending balance $ (899) $ (871)