Investment and derivative contract gains/losses - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Schedule Of Gain Loss On Investments Including Marketable Securities And Investments Held At Cost Income Statement Reported Amounts Summary [Line Items]
Sales of equity securities			$ 11,975	$ 28,564
Taxable gains on sales of equity securities	$ 941	$ 3,900	$ 2,900	$ 666
Not Designated as Hedging Instrument [Member] | Equity Index Put Options [Member]
Schedule Of Gain Loss On Investments Including Marketable Securities And Investments Held At Cost Income Statement Reported Amounts Summary [Line Items]
Derivative maturity month and year			2023-02
Premiums received at contract inception dates	$ 1,500		$ 1,500