Investments in fixed maturity securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Summary of Investment Holdings [Line Items]
Unrealized Gains on Investments	$ 36,732	$ 28,773
Unrealized Losses on Investments	(274)	(1,392)
Fixed Maturities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	29,359	30,048
Unrealized Gains on Investments	2,570	2,358
Unrealized Losses on Investments	(231)	(218)
Fair Value of Investments	31,698	32,188
Fixed Maturities [Member] \| US Government and Government Agencies and Authorities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,621	2,894
Unrealized Gains on Investments	34	41
Unrealized Losses on Investments	(3)
Fair Value of Investments	2,652	2,935
Fixed Maturities [Member] \| States, municipalities and political subdivisions [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,749	2,862
Unrealized Gains on Investments	202	208
Fair Value of Investments	2,951	3,070
Fixed Maturities [Member] \| Foreign governments [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	10,738	10,608
Unrealized Gains on Investments	281	283
Unrealized Losses on Investments	(28)	(48)
Fair Value of Investments	10,991	10,843
Fixed Maturities [Member] \| Corporate bonds [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	10,772	11,120
Unrealized Gains on Investments	1,713	1,483
Unrealized Losses on Investments	(188)	(155)
Fair Value of Investments	12,297	12,448
Fixed Maturities [Member] \| Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized Cost of Investments	2,479	2,564
Unrealized Gains on Investments	340	343
Unrealized Losses on Investments	(12)	(15)
Fair Value of Investments	$ 2,807	$ 2,892

Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

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Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 10: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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+ Details

- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References+ Details