Investments - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($) security	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($) security	Dec. 31, 2017 USD ($) security
Gain (Loss) on Investments [Line Items]
Largest Single Common Stock Holding	$ 316,000,000		$ 316,000,000
Largest equity holdings to total equity portfolio	4.90%		4.90%
Largest equity holdings to total portfolio	1.80%		1.80%
Unrealized losses 12 months or more (equity investment less than $500,000)					$ (18,000,000)
Fixed maturity investments, Percentage of amortized costs			70.00%		70.00%
Fixed maturities
Gain (Loss) on Investments [Line Items]
Number Of Securities Other Than Temporary Impairment Losses1 | security	0	0	0	1
Provision for Other Credit Losses	$ 0	$ 0	$ 0	$ 0
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security	288		288		249
Unrealized losses 12 months or more (equity investment less than $500,000)	$ (49,000,000)		$ (49,000,000)		$ (18,000,000)
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost	0		0		0
Equity securities
Gain (Loss) on Investments [Line Items]
Number Of Securities Other Than Temporary Impairment Losses1 | security		0		5
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months					0
Unrealized losses 12 months or more (equity investment less than $500,000)					$ 0