Pension plans - Net periodic pension expense (Detail) (Pension Plans, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Plans, Defined Benefit [Member]
Components of net periodic pension expense
Service cost	$ 247	$ 191	$ 165
Interest cost	583	568	543
Expected return on plan assets	(610)	(579)	(528)
Other, primarily amortization of actuarial losses	220	102	69
Net pension expense	$ 440	$ 282	$ 249