Investments - Additional Information (Detail) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($) security	Mar. 31, 2020 security	Dec. 31, 2020 USD ($) security
Gain (Loss) on Securities
Debt Securities, Available-for-sale, Allowance for Credit Loss | security	0		0
Equity securities, at fair value	$ 9,360		$ 8,856
Largest Single Common Stock Holding	$ 593		$ 644
Largest equity holdings to total equity portfolio	6.60%		7.50%
Largest equity holdings to total portfolio	2.70%		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	0	12
Number Of Securities Continuous Unrealized Loss Position | security	296		128
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss 3	$ 19		$ 10
Number Of Securities Continuous Unrealized Loss Position and Below 70 Percent of Amortized Cost	1		0
Common equities | Equity securities
Gain (Loss) on Securities
Equity securities, at fair value	$ 9,032		$ 8,541