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Pension plans - Defined benefit pension plans - Narrative (Detail) - Pension Plans, Defined Benefit [Member] - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Projected benefit obligation $ 20,429 $ 18,706 $ 16,922
Amounts recognized in the Consolidated Balance Sheets:      
Funded status reflected in assets 1,351 857  
Funded status reflected in liabilities 3,842 3,067  
ABO 19,400 17,500  
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]      
2021 1,105    
2022 1,031    
2023 1,034    
2024 1,037    
2025 1,040    
2026 - 2030 5,119    
Expected contribution to pension plans during the next year 202    
Unfunded Plans [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Projected benefit obligation $ 1,600 $ 1,300