Investments - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) security	Jun. 30, 2018 USD ($) security	Jun. 30, 2019 USD ($) security	Jun. 30, 2018 USD ($) security	Dec. 31, 2018 USD ($) security
Gain (Loss) on Securities [Line Items]
Largest Single Common Stock Holding	$ 336,000,000		$ 336,000,000
Largest equity holdings to total equity portfolio	4.90%		4.90%
Largest equity holdings to total portfolio	1.80%		1.80%
Fixed maturity investments, Percentage of amortized costs			70.00%		70.00%
Fixed maturities
Gain (Loss) on Securities [Line Items]
Number Of Securities Other Than Temporary Impairment Losses1 | security	0	0	0	0
Provision for Other Credit Losses	$ 0	$ 0	$ 0	$ 0
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security	100		100		400
Unrealized losses 12 months or more (equity investment less than $500,000)	$ (12,000,000)		$ (12,000,000)		$ (58,000,000)
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost	1		1		0