Investment gains/losses - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Sales of equity securities	$ 38,756	$ 14,336	$ 18,783
Taxable gains (loss) on sales of equity securities	$ 6,200	$ 3,200	$ 3,300