Investments in fixed maturity securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Summary of Investment Holdings [Line Items]
Unrealized Gains on Investments	$ 46,210	$ 37,357
Unrealized Losses on Investments	(122)	(202)
Fixed Maturities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	28,568	29,261
Unrealized Gains on Investments	2,885	3,085
Unrealized Losses on Investments	(70)	(55)
Fair Value of Investments	31,383	32,291
Fixed Maturities [Member] | U.S. Treasury, U.S. government corporations and agencies [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,886	2,742
Unrealized Gains on Investments	31	33
Unrealized Losses on Investments	(1)
Fair Value of Investments	2,916	2,775
Fixed Maturities [Member] | States, municipalities and political subdivisions [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,654	2,735
Unrealized Gains on Investments	169	178
Unrealized Losses on Investments	(1)
Fair Value of Investments	2,822	2,913
Fixed Maturities [Member] | Foreign governments [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	10,846	11,098
Unrealized Gains on Investments	252	302
Unrealized Losses on Investments	(61)	(45)
Fair Value of Investments	11,037	11,355
Fixed Maturities [Member] | Corporate bonds [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	10,054	10,410
Unrealized Gains on Investments	2,129	2,254
Unrealized Losses on Investments	(3)	(3)
Fair Value of Investments	12,180	12,661
Fixed Maturities [Member] | Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,128	2,276
Unrealized Gains on Investments	304	318
Unrealized Losses on Investments	(4)	(7)
Fair Value of Investments	$ 2,428	$ 2,587