Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Balance at beginning of year	$ 62,708	$ 60,714
Acquisitions of businesses	16,617	2,563
Other, including foreign currency translation	(211)	(569)
Balance at end of period	$ 79,114	$ 62,708