Investments in fixed maturity securities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Summary of Investment Holdings [Line Items]
Unrealized Gains on Investments	$ 35,710	$ 28,773
Unrealized Losses on Investments	(688)	(1,392)
Fixed Maturities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	28,834	30,048
Unrealized Gains on Investments	2,712	2,358
Unrealized Losses on Investments	(111)	(218)
Fair Value of Investments	31,435	32,188
Fixed Maturities [Member] | US Government and Government Agencies and Authorities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,594	2,894
Unrealized Gains on Investments	36	41
Fair Value of Investments	2,630	2,935
Fixed Maturities [Member] | States, municipalities and political subdivisions [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,716	2,862
Unrealized Gains on Investments	194	208
Fair Value of Investments	2,910	3,070
Fixed Maturities [Member] | Foreign governments [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	10,917	10,608
Unrealized Gains on Investments	303	283
Unrealized Losses on Investments	(84)	(48)
Fair Value of Investments	11,136	10,843
Fixed Maturities [Member] | Corporate bonds [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	10,234	11,120
Unrealized Gains on Investments	1,836	1,483
Unrealized Losses on Investments	(16)	(155)
Fair Value of Investments	12,054	12,448
Fixed Maturities [Member] | Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,373	2,564
Unrealized Gains on Investments	343	343
Unrealized Losses on Investments	(11)	(15)
Fair Value of Investments	$ 2,705	$ 2,892