Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Balance at beginning of year	$ 81,258	$ 79,486
Acquisitions of businesses	52	1,545
Other, including foreign currency translation	27	227
Balance at end of period	$ 81,337	$ 81,258