Investments in fixed maturity securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Summary of Investment Holdings [Line Items]
Unrealized Gains on Investments	$ 48,373	$ 37,357
Unrealized Losses on Investments	(333)	(202)
Fixed Maturities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	27,213	29,261
Unrealized Gains on Investments	2,418	3,085
Unrealized Losses on Investments	(96)	(55)
Fair Value of Investments	29,535	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	2,537	2,742
Unrealized Gains on Investments	21	33
Unrealized Losses on Investments	(6)
Fair Value of Investments	2,552	2,775
Fixed Maturities [Member] | States, municipalities and political subdivisions [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,397	2,735
Unrealized Gains on Investments	141	178
Unrealized Losses on Investments	(5)
Fair Value of Investments	2,533	2,913
Fixed Maturities [Member] | Foreign governments [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	10,225	11,098
Unrealized Gains on Investments	213	302
Unrealized Losses on Investments	(68)	(45)
Fair Value of Investments	10,370	11,355
Fixed Maturities [Member] | Corporate bonds [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	10,136	10,410
Unrealized Gains on Investments	1,806	2,254
Unrealized Losses on Investments	(10)	(3)
Fair Value of Investments	11,932	12,661
Fixed Maturities [Member] | Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	1,918	2,276
Unrealized Gains on Investments	237	318
Unrealized Losses on Investments	(7)	(7)
Fair Value of Investments	$ 2,148	$ 2,587