Investments in fixed maturity securities (Detail) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Investments in securities with fixed maturities [Line Items]
Amortized Cost	$ 31,761	$ 31,839
Unrealized Gains	2,710	3,131
Unrealized Losses	(423)	(87)
Fair Value	34,048	34,883
Fixed Maturities [Member] | U.S. Treasury, U.S. government corporations and agencies [Member]
Investments in securities with fixed maturities [Line Items]
Amortized Cost	2,245	2,151
Unrealized Gains	47	48
Unrealized Losses		(2)
Fair Value	2,292	2,197
Fixed Maturities [Member] | States, municipalities and political subdivisions [Member]
Investments in securities with fixed maturities [Line Items]
Amortized Cost	2,938	3,356
Unrealized Gains	216	225
Fair Value	3,154	3,581
Fixed Maturities [Member] | Foreign governments [Member]
Investments in securities with fixed maturities [Line Items]
Amortized Cost	12,972	11,721
Unrealized Gains	286	242
Unrealized Losses	(62)	(51)
Fair Value	13,196	11,912
Fixed Maturities [Member] | Corporate bonds [Member]
Investments in securities with fixed maturities [Line Items]
Amortized Cost	11,142	11,773
Unrealized Gains	1,804	2,304
Unrealized Losses	(345)	(23)
Fair Value	12,601	14,054
Fixed Maturities [Member] | Mortgage-backed securities [Member]
Investments in securities with fixed maturities [Line Items]
Amortized Cost	2,464	2,838
Unrealized Gains	357	312
Unrealized Losses	(16)	(11)
Fair Value	$ 2,805	$ 3,139