Investments - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) security	Sep. 30, 2020 security	Sep. 30, 2021 USD ($) security	Sep. 30, 2020 security	Dec. 31, 2020 USD ($) security
Gain (Loss) on Securities
Debt Securities, Available-for-sale, Allowance for Credit Loss | security	0		0		0
Equity Securities at Fair Value | $	$ 9,887		$ 9,887		$ 8,856
Largest Single Common Stock Holding | $	$ 687		$ 687		$ 644
Largest equity holdings to total equity portfolio	7.30%		7.30%		7.50%
Largest equity holdings to total portfolio	3.00%		3.00%		3.00%
Fixed maturity investments, Percentage of amortized costs			70.00%		70.00%
Fixed maturities
Gain (Loss) on Securities
Number Of Impaired Securities Intended to be Sold | security	5	2	5	14
Number Of Securities Continuous Unrealized Loss Position | security	244		244		128
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss 3 | $	$ 12		$ 12		$ 10
Number Of Securities Continuous Unrealized Loss Position and Below 70 Percent of Amortized Cost	1		1		0