Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Balance at beginning of year	$ 79,486	$ 62,708
Acquisitions of businesses	1,545	17,650
Other, including foreign currency translation	227	(872)
Balance at end of year	$ 81,258	$ 79,486