Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Jun. 30, 2020	Dec. 31, 2020		Dec. 31, 2019
Goodwill And Intangible Assets Disclosure [Abstract]
Balance at beginning of year			$ 81,882	[1]	$ 81,025
Acquisitions of businesses			1,758		890
Impairment charges		$ (10,000)	(10,033)		(90)
Other, including foreign currency translation			127		57
Balance at end of year	[1]		$ 73,734		$ 81,882

[1] Net of accumulated goodwill impairments of $11.0 billion as of December 31, 2020 and $1.1 billion as of December 31, 2019.