Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Balance at beginning of year	$ 81,258	$ 79,486
Acquisitions of businesses	376	1,545
Other, including foreign currency translation	(609)	227
Balance at end of year	$ 81,025	$ 81,258