Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Goodwill And Intangible Assets Disclosure [Abstract]
Balance at beginning of year		$ 81,882	$ 81,025
Acquisitions of businesses		18	890
Impairment charges	$ (10,000)	(9,996)
Other, including foreign currency translation		(151)	(33)
Balance at end of period	$ 71,753	$ 71,753	$ 81,882