Pension plans - Net periodic pension expense (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of net periodic pension expense
Service cost	$ 224	$ 271	$ 273
Interest cost	618	593	635
Expected return on plan assets	(936)	(988)	(939)
Amortization of actuarial losses and other	26	188	157
Net periodic pension expense	$ (68)	$ 64	$ 126