Pension plans - Net periodic pension expense (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of net periodic pension expense
Service cost	$ 235	$ 224	$ 271
Interest cost	510	618	593
Expected return on plan assets	(955)	(936)	(988)
Amortization of actuarial losses and other	171	26	188
Net periodic pension expense	$ (39)	$ (68)	$ 64