Pension plans - Defined benefit pension plans - Narrative (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 18,706	$ 16,922	$ 18,824
Amounts recognized in the Consolidated Balance Sheets:
Funded status reflected in assets	857	510
Funded status reflected in liabilities	3,067	2,473
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2020	1,059
2021	997
2022	1,003
2023	1,009
2024	1,017
2025 - 2029	5,035
Expected contribution to pension plans during the next year	191
Unfunded Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 1,300	$ 1,200