Investments - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) security	Mar. 31, 2017 USD ($) security	Dec. 31, 2017 USD ($) security
Gain (Loss) on Investments [Line Items]
Largest Single Common Stock Holding	$ 248,000,000
Largest equity holdings to total equity portfolio	4.20%
Largest equity holdings to total portfolio	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	0
Provision for Other Credit Losses	$ 0	$ 0
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security	250		249
Unrealized losses 12 months or more (equity investment less than $500,000)	$ (34,000,000)		$ (18,000,000)
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost	1		0
Equity securities
Gain (Loss) on Investments [Line Items]
Number Of Securities Other Than Temporary Impairment Losses1 | security		0
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