Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Goodwill And Intangible Assets Disclosure [Abstract]
Balance at beginning of year		$ 81,882	$ 81,025
Acquisitions of businesses		25	890
Impairment charges	$ (10,000)	(10,021)
Other, including foreign currency translation		(21)	(33)
Balance at end of period		$ 71,865	$ 81,882