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Post-retirement benefits - Summary of movement in net defined benefit liability (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance $ (1,730) $ (1,551)  
Amounts recognised in income statement (205) (146) $ (288)
Amounts recognised in other comprehensive income (474) (262) 907
Employer contributions 201 257 248
Arrangements added/divested 0 (5)  
Assets transferred to defined contribution section (3) (3)  
Currency exchange rate (loss) (62) (20)  
Ending balance (2,273) (1,730) $ (1,551)
Pension benefits      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (831)    
Amounts recognised in income statement (172)    
Amounts recognised in other comprehensive income (416)    
Employer contributions 169    
Arrangements added/divested 0    
Assets transferred to defined contribution section (3)    
Currency exchange rate (loss) (52)    
Ending balance (1,305) (831)  
Other Benefits      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (899)    
Amounts recognised in income statement (33)    
Amounts recognised in other comprehensive income (58)    
Employer contributions 32    
Arrangements added/divested 0    
Assets transferred to defined contribution section 0    
Currency exchange rate (loss) (10)    
Ending balance $ (968) $ (899)