Pension plans - Net periodic pension expense (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Components of net periodic pension expense
Service cost	$ 266	$ 230	$ 254
Interest cost	591	629	547
Expected return on plan assets	(782)	(772)	(634)
Amortization of actuarial losses and other	179	102	225
Net pension expense	$ 254	$ 189	$ 392