Goodwill and other intangible assets - Intangible assets - Narrative (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013 Finite-lived Intangible Assets [Member]	Sep. 30, 2012 Finite-lived Intangible Assets [Member]	Sep. 30, 2013 Unclassified Indefinite-lived Intangible Assets [Member]	Dec. 31, 2012 Unclassified Indefinite-lived Intangible Assets [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Amortization expense	$ 797	$ 760
Intangible assets with indefinite lives, excluding goodwill			$ 2,344	$ 2,328