Investments - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($) security	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($) security	Dec. 31, 2017 USD ($) security
Gain (Loss) on Investments [Line Items]
Largest Single Common Stock Holding	$ 259,000,000		$ 259,000,000
Largest equity holdings to total equity portfolio	4.30%		4.30%
Largest equity holdings to total portfolio	1.50%		1.50%
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	1	0	1
Provision for Other Credit Losses	$ 0	$ 0	$ 0	$ 0
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security	263		263		249
Unrealized losses 12 months or more (equity investment less than $500,000)	$ (40,000,000)		$ (40,000,000)		$ (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		5		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