Pension plans - Net periodic pension expense (Detail) (Pension Plans, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Pension Plans, Defined Benefit [Member]
Components of net periodic pension expense
Service cost	$ 191	$ 165	$ 162
Interest cost	568	543	455
Expected return on plan assets	(579)	(528)	(417)
Other periodic pension expense	102	69	35
Net pension expense	$ 282	$ 249	$ 235