Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Balance at beginning of year	$ 81,258	$ 79,486
Acquisitions of businesses	177	1,545
Other, including foreign currency translation	(185)	227
Balance at end of period	$ 81,250	$ 81,258