Investments - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) security	Sep. 30, 2019 security	Sep. 30, 2020 USD ($) security	Sep. 30, 2019 security	Dec. 31, 2019 USD ($) security
Gain (Loss) on Securities
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0
Equity securities, at fair value | $	$ 7,867		$ 7,867		$ 7,752
Largest Single Common Stock Holding | $	$ 562		$ 562
Largest equity holdings to total equity portfolio	7.40%		7.40%
Largest equity holdings to total portfolio	2.80%		2.80%
Number Of Securities Other Than Temporary Impairment Losses1		2
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	2		14
Number Of Securities Other Than Temporary Impairment Losses1				2
Number Of Securities Continuous Unrealized Loss Position	254		254
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss 3 | $	$ 32		$ 32		$ 7
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months					38
Unrealized losses 12 months or more (equity investment less than $500,000) | $	$ (4)		$ (4)		$ (3)
Number Of Securities Continuous Unrealized Loss Position and Below 70 Percent of Amortized Cost	1		1
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost					0
Common equities | Equity securities
Gain (Loss) on Securities
Equity securities, at fair value | $	$ 7,589		$ 7,589		$ 7,518