Investments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) Investment	Dec. 31, 2017 USD ($) Investment	Dec. 31, 2016
Schedule of Investments [Line Items]
Value of non-income producing fixed maturity securities held	$ 0	$ 2,000,000
Number of Investments not exceeds 10% of shareholders equity | Investment	0	0
Additional other than temporary impairment losses	$ 0
Carrying value of limited partnerships	2,400,000,000	$ 3,300,000,000
Net undistributed earnings of limited partnerships	$ 208,000,000	$ 903,000,000
Percentage of carrying value reported on a current basis	67.60%
Percentage of carrying value reported on one month lag	11.40%
Percentage of limited partnerships comprising of the carrying value that employ hedge fund strategies	71.30%	78.80%
Percentage of limited partnerships comprising of the carrying value that invested in private debt and equity	24.30%	18.10%
Percentage of limited partnerships employing hedge fund strategies focused on equity investments	55.90%
Percentage of limited partnerships employing hedge fund strategies with a multi-strategy approach	26.40%
Percentage of limited partnerships employing hedge fund strategies focused on distressed investments	14.40%
Percentage of limited partnerships employing hedge fund strategies focused on fixed income investments	3.30%
Ownership percentage of aggregate partnership equity	2.60%	2.90%
Changes in ownership percentage of aggregate partnership equity	3.30%	3.00%	4.00%
Withdrawal provisions limited partnership investments, description	The Company's limited partnership investments contain withdrawal provisions that generally limit liquidity for a period of thirty days up to one year and in some cases do not permit withdrawals until the termination of the partnership. Typically, withdrawals require advance written notice of up to 90 days.
Future capital call commitments	$ 574,000,000
Commitments to purchase various privately placed debt securities, including bank loans	230,000,000
Commitments to sell various privately placed debt securities, including bank loans	78,000,000
Securities deposited	2,500,000,000	$ 2,600,000,000
Cash and securities with carrying values deposited as collateral for letters of credit	1,000,000,000	1,100,000,000
Ten Largest Limited Partnership Holdings [Member]
Schedule of Investments [Line Items]
Carrying value of limited partnerships	$ 1,100,000,000	$ 1,500,000,000