Pension plans - Net periodic pension expense (Detail) (Pension Plans, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Plans, Defined Benefit [Member]
Components of net periodic pension expense
Service cost	$ 230	$ 254	$ 247
Interest cost	629	547	583
Expected return on plan assets	(772)	(634)	(610)
Amortization of actuarial losses and other	102	225	220
Net pension expense	$ 189	$ 392	$ 440