Pension plans - Net periodic pension expense (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of net periodic pension expense
Service cost	$ 271	$ 273	$ 282
Interest cost	593	635	691
Expected return on plan assets	(988)	(939)	(908)
Amortization of actuarial losses and other	188	157	148
Net periodic pension expense	$ 64	$ 126	$ 213