Investments in fixed maturity securities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	$ 22,434	$ 25,049
Unrealized gains on fixed maturity securities	1,121	1,107
Unrealized losses on fixed maturity securities	(90)	(129)
Fair value of fixed maturity securities	23,465	26,027
U.S. Treasury, U.S. government corporations and agencies [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	4,519	3,425
Unrealized gains on fixed maturity securities	16	10
Unrealized losses on fixed maturity securities	(8)	(8)
Fair value of fixed maturity securities	4,527	3,427
States, municipalities and political subdivisions [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	1,159	1,695
Unrealized gains on fixed maturity securities	58	71
Unrealized losses on fixed maturity securities	(1)	(2)
Fair value of fixed maturity securities	1,216	1,764
Foreign governments [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	8,860	11,327
Unrealized gains on fixed maturity securities	207	226
Unrealized losses on fixed maturity securities	(66)	(85)
Fair value of fixed maturity securities	9,001	11,468
Corporate bonds [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	6,899	7,323
Unrealized gains on fixed maturity securities	714	632
Unrealized losses on fixed maturity securities	(9)	(29)
Fair value of fixed maturity securities	7,604	7,926
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	997	1,279
Unrealized gains on fixed maturity securities	126	168
Unrealized losses on fixed maturity securities	(6)	(5)
Fair value of fixed maturity securities	$ 1,117	$ 1,442

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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