Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions		3 Months Ended	9 Months Ended		12 Months Ended
		Jun. 30, 2020	Sep. 30, 2021		Dec. 31, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
Balance at beginning of year			$ 73,734	[1]	$ 81,882
Acquisitions of businesses			182		1,758
Impairment charges		$ (10,000)			(10,033)
Other, including foreign currency translation			(146)		127
Balance at end of period	[1]		$ 73,770		$ 73,734

[1] Net of accumulated goodwill impairments of $11.0 billion as of September 30, 2021 and December 31, 2020.