Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions		3 Months Ended			12 Months Ended
		Mar. 31, 2021		Jun. 30, 2020	Dec. 31, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
Balance at beginning of year		$ 73,734	[1]		$ 81,882
Acquisitions of businesses		7			1,758
Impairment charges				$ (10,000)	(10,033)
Other, including foreign currency translation		(46)			127
Balance at end of period	[1]	$ 73,695			$ 73,734

[1] Net of accumulated goodwill impairments of $11.0 billion as of March 31, 2021 and  December 31, 2020.