Investments - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($) security	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($) security	Dec. 31, 2018 USD ($) security
Gain (Loss) on Securities
Largest Single Common Stock Holding	$ 349,000,000		$ 349,000,000
Largest equity holdings to total equity portfolio	5.00%		5.00%
Largest equity holdings to total portfolio	1.90%		1.90%
Fixed maturity investments, Percentage of amortized costs			70.00%		70.00%
Fixed maturities
Gain (Loss) on Securities
Number Of Securities Other Than Temporary Impairment Losses1 | security	2	0	2	0
Provision for Other Credit Losses	$ 0	$ 0	$ 0	$ 0
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security	62		62		400
Unrealized losses 12 months or more (equity investment less than $500,000)	$ (9,000,000)		$ (9,000,000)		$ (58,000,000)
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost	2		2		0