Pension plans - Net periodic pension expense (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of net periodic pension expense
Service cost	$ 282	$ 266	$ 230
Interest cost	691	591	629
Expected return on plan assets	(908)	(782)	(772)
Amortization of actuarial losses and other	148	179	102
Net pension expense	$ 213	$ 254	$ 189