Investment and derivative contract gains/losses - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 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	$ 6,451	$ 2,166
Taxable gains (loss) on sales of equity securities	$ 1,800	$ 1,200
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