Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Balance at beginning of year	$ 81,258	$ 79,486
Acquisitions of businesses	250	1,545
Other, including foreign currency translation	(181)	227
Balance at end of period	$ 81,327	$ 81,258