Components of Net Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income									$ 3,309	$ 3,033	$ 2,789
Less investment expenses									29	26	24
Net investment income	858	843	784	794	789	765	727	726	3,280	3,007	2,765
Fixed Maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income									3,026	2,695	2,413
Perpetual Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income									274	328	367
Equity Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income									5	4	3
Short Term Investments and Cash Equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income									$ 4	$ 6	$ 6