Investments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) Investment	Dec. 31, 2017
Schedule of Investments [Line Items]
Number of Investments not exceeds 10% of shareholders equity | Investment		0
Additional other than temporary impairment losses	$ 0
Carrying value of limited partnerships	2,000,000,000.0	$ 2,400,000,000
Net undistributed earnings of limited partnerships	$ 297,000,000	$ 208,000,000
Percentage of carrying value reported on a current basis	64.40%
Percentage of carrying value reported on one month lag	9.10%
Percentage of limited partnerships comprising of the carrying value that employ hedge fund strategies	65.70%	71.30%
Percentage of limited partnerships comprising of the carrying value that invested in private debt and equity	29.10%	24.30%
Percentage of limited partnerships employing hedge fund strategies focused on equity investments	44.00%
Percentage of limited partnerships employing hedge fund strategies with a multi-strategy approach	32.30%
Percentage of limited partnerships employing hedge fund strategies focused on distressed investments	18.80%
Percentage of limited partnerships employing hedge fund strategies focused on fixed income investments	4.90%
Ownership percentage of aggregate partnership equity	2.30%	2.60%
Changes in ownership percentage of aggregate partnership equity	2.50%	3.30%	3.00%
Withdrawal provisions limited partnership investments, description	The Company’s hedge fund limited partnership investments contain withdrawal provisions that generally limit liquidity for a period of thirty days up to one year or longer. Private equity and other non-hedge funds generally do not permit voluntary withdrawals. Typically, hedge fund withdrawals require advance written notice of up to 90 days.
Commitments to purchase various privately placed debt securities, including bank loans	$ 945,000,000
Commitments to sell various privately placed debt securities, including bank loans	85,000,000
Securities deposited	2,700,000,000	$ 2,500,000,000
Cash and securities with carrying values deposited as collateral for letters of credit	1,100,000,000	1,000,000,000.0
Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Non-income producing fixed maturity securities	1,000,000	0
Ten Largest Limited Partnership Holdings
Schedule of Investments [Line Items]
Carrying value of limited partnerships	$ 1,100,000,000	$ 1,100,000,000