Pension plans - Net periodic pension expense (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of net periodic pension expense
Service cost	$ 273	$ 282	$ 266
Interest cost	635	691	591
Expected return on plan assets	(939)	(908)	(782)
Amortization of actuarial losses and other	157	148	179
Net periodic pension expense	$ 126	$ 213	$ 254