Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Balance at beginning of year	$ 60,714	$ 57,011
Acquisitions of businesses	2,388	4,006
Other, including foreign currency translation	(410)	(303)
Balance at end of period	$ 62,692	$ 60,714