Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Balance at beginning of year	$ 81,025	$ 81,258
Acquisitions of businesses	159	376
Other, including foreign currency translation	36	(609)
Balance at end of period	$ 81,220	$ 81,025