Investments in fixed maturity securities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Summary of Investment Holdings [Line Items]
Unrealized Gains on Investments	$ 28,773	$ 26,641
Unrealized Losses on Investments	(1,392)	(672)
Fixed Maturities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	30,048	31,839
Unrealized Gains on Investments	2,358	3,131
Unrealized Losses on Investments	(218)	(87)
Fair Value of Investments	32,188	34,883
Fixed Maturities [Member] | US Government and Government Agencies and Authorities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,894	2,151
Unrealized Gains on Investments	41	48
Unrealized Losses on Investments		(2)
Fair Value of Investments	2,935	2,197
Fixed Maturities [Member] | States, municipalities and political subdivisions [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,862	3,356
Unrealized Gains on Investments	208	225
Fair Value of Investments	3,070	3,581
Fixed Maturities [Member] | Foreign governments [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	10,608	11,721
Unrealized Gains on Investments	283	242
Unrealized Losses on Investments	(48)	(51)
Fair Value of Investments	10,843	11,912
Fixed Maturities [Member] | Corporate bonds [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	11,120	11,773
Unrealized Gains on Investments	1,483	2,304
Unrealized Losses on Investments	(155)	(23)
Fair Value of Investments	12,448	14,054
Fixed Maturities [Member] | Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,564	2,838
Unrealized Gains on Investments	343	312
Unrealized Losses on Investments	(15)	(11)
Fair Value of Investments	$ 2,892	$ 3,139