Investments in fixed maturity securities (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	$ 25,049	$ 26,023
Unrealized gains on fixed maturity securities	1,107	1,754
Unrealized losses on fixed maturity securities	(129)	(141)
Fair value of fixed maturity securities	26,027	27,636
U.S. Treasury, U.S. government corporations and agencies [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	3,425	2,921
Unrealized gains on fixed maturity securities	10	14
Unrealized losses on fixed maturity securities	(8)	(5)
Fair value of fixed maturity securities	3,427	2,930
States, municipalities and political subdivisions [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	1,695	1,820
Unrealized gains on fixed maturity securities	71	93
Unrealized losses on fixed maturity securities	(2)	(1)
Fair value of fixed maturity securities	1,764	1,912
Foreign governments [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	11,327	12,023
Unrealized gains on fixed maturity securities	226	373
Unrealized losses on fixed maturity securities	(85)	(126)
Fair value of fixed maturity securities	11,468	12,270
Corporate bonds [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	7,323	7,704
Unrealized gains on fixed maturity securities	632	1,072
Unrealized losses on fixed maturity securities	(29)	(5)
Fair value of fixed maturity securities	7,926	8,771
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	1,279	1,555
Unrealized gains on fixed maturity securities	168	202
Unrealized losses on fixed maturity securities	(5)	(4)
Fair value of fixed maturity securities	$ 1,442	$ 1,753

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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