Investments in fixed maturity securities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Summary of Investment Holdings [Line Items]
Unrealized Gains on Investments	$ 47,808	$ 37,357
Unrealized Losses on Investments	(337)	(202)
Fixed Maturities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	27,221	29,261
Unrealized Gains on Investments	2,462	3,085
Unrealized Losses on Investments	(178)	(55)
Fair Value of Investments	29,505	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	2,799	2,742
Unrealized Gains on Investments	22	33
Unrealized Losses on Investments	(10)
Fair Value of Investments	2,811	2,775
Fixed Maturities [Member] | States, municipalities and political subdivisions [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,600	2,735
Unrealized Gains on Investments	152	178
Unrealized Losses on Investments	(4)
Fair Value of Investments	2,748	2,913
Fixed Maturities [Member] | Foreign governments [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	10,222	11,098
Unrealized Gains on Investments	156	302
Unrealized Losses on Investments	(140)	(45)
Fair Value of Investments	10,238	11,355
Fixed Maturities [Member] | Corporate bonds [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	9,600	10,410
Unrealized Gains on Investments	1,880	2,254
Unrealized Losses on Investments	(16)	(3)
Fair Value of Investments	11,464	12,661
Fixed Maturities [Member] | Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,000	2,276
Unrealized Gains on Investments	252	318
Unrealized Losses on Investments	(8)	(7)
Fair Value of Investments	$ 2,244	$ 2,587