Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Balance at beginning of year	$ 79,486	$ 62,708
Acquisitions of businesses	540	17,650
Other, including foreign currency translation	(75)	(872)
Balance at end of period	$ 79,951	$ 79,486