Investments - Additional Information (Detail)		12 Months Ended
	Dec. 31, 2015 USD ($) Investment	Dec. 31, 2015 USD ($) Investment	Dec. 31, 2014 USD ($) Investment	Dec. 31, 2013
Schedule of Investments [Line Items]
Value of non-income producing fixed maturity securities held	$ 54,000,000	$ 54,000,000	$ 0
Number of Investments not exceeds 10% of shareholders equity | Investment	0	0	0
Fixed maturities, fair value	$ 39,701,000,000	$ 39,701,000,000	$ 40,885,000,000
Fixed maturities, amortized cost	37,407,000,000	37,407,000,000	37,469,000,000
Equity securities, fair value	752,000,000	752,000,000	728,000,000
Equity securities, amortized cost	824,000,000	824,000,000	733,000,000
Fair market value of marketable securities unrealized loss position	1,400,000,000	1,400,000,000
Amortized cost of marketable securities unrealized loss position	1,600,000,000	1,600,000,000
Additional other than temporary impairment losses	0
Carrying value of limited partnerships	$ 3,300,000,000	3,300,000,000	3,700,000,000
Undistributed earnings of limited partnerships		$ 952,000,000	$ 1,300,000,000
Percentage of carrying value reported on a current basis		70.80%
Percentage of carrying value reported on one month lag		12.80%
Percentage of limited partnerships comprising of the carrying value that employ hedge fund strategies	76.60%	76.60%	78.60%
Percentage of limited partnerships comprising of the carrying value that invested in private debt and equity	23.40%	23.40%	18.60%
Percentage of limited partnerships employing hedge fund strategies focused on equity investments	56.40%	56.40%
Percentage of limited partnerships employing hedge fund strategies with a multi-strategy approach	28.90%	28.90%
Percentage of limited partnerships employing hedge fund strategies focused on distressed investments	11.40%	11.40%
Percentage of limited partnerships employing hedge fund strategies focused on fixed income investments	3.30%	3.30%
Ownership percentage of aggregate partnership equity	2.80%	2.80%	3.90%
Changes in ownership percentage of aggregate partnership equity		2.80%	4.30%	3.70%
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	$ 398,000,000	$ 398,000,000
Commitments to purchase various privately placed debt securities, including bank loans	138,000,000	138,000,000
Commitments to sell various privately placed debt securities, including bank loans	67,000,000	67,000,000
Securities deposited	2,800,000,000	2,800,000,000	$ 3,000,000,000
Cash and securities with carrying values deposited as collateral for letters of credit	364,000,000	364,000,000	361,000,000
Securities deposited as collateral for letters of credit	263,000,000	263,000,000	302,000,000
Energy, Metals and Mining Sector [Member]
Schedule of Investments [Line Items]
Fixed maturities, fair value	2,500,000,000	2,500,000,000
Fixed maturities, amortized cost	2,700,000,000	2,700,000,000
Equity securities, fair value	2,500,000,000	2,500,000,000
Equity securities, amortized cost	2,700,000,000	2,700,000,000
Ten Largest Limited Partnership Holdings [Member]
Schedule of Investments [Line Items]
Carrying value of limited partnerships	$ 1,500,000,000	$ 1,500,000,000	$ 1,800,000,000