Pension plans - Net periodic pension expense (Detail) (Pension Plans, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Pension Plans, Defined Benefit [Member]
Components of net periodic pension expense
Service cost	$ 254	$ 247	$ 191
Interest cost	547	583	568
Expected return on plan assets	(634)	(610)	(579)
Amortization of actuarial losses and other	225	220	102
Net pension expense	$ 392	$ 440	$ 282