Investment and derivative contract gains/losses - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 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			$ 8,596	$ 15,743
Taxable gains (loss) on sales of equity securities	$ 228	$ (4,500)	$ 2,000	$ (3,300)
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	$ 1,900		$ 1,900