Investments in fixed maturity securities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	$ 20,445	$ 22,404
Unrealized gains on fixed maturity securities	977	1,118
Unrealized losses on fixed maturity securities	(69)	(90)
Fair value of fixed maturity securities	21,353	23,432
U.S. Treasury, U.S. government corporations and agencies [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	3,975	4,519
Unrealized gains on fixed maturity securities	4	16
Unrealized losses on fixed maturity securities	(26)	(8)
Fair value of fixed maturity securities	3,953	4,527
States, municipalities and political subdivisions [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	847	1,159
Unrealized gains on fixed maturity securities	19	58
Unrealized losses on fixed maturity securities	(12)	(1)
Fair value of fixed maturity securities	854	1,216
Foreign governments [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	8,572	8,860
Unrealized gains on fixed maturity securities	274	207
Unrealized losses on fixed maturity securities	(24)	(66)
Fair value of fixed maturity securities	8,822	9,001
Corporate bonds [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	6,279	6,899
Unrealized gains on fixed maturity securities	588	714
Unrealized losses on fixed maturity securities	(5)	(9)
Fair value of fixed maturity securities	6,862	7,604
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	772	967
Unrealized gains on fixed maturity securities	92	123
Unrealized losses on fixed maturity securities	(2)	(6)
Fair value of fixed maturity securities	$ 862	$ 1,084

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
-Topic 320
-SubTopic 10
-Section 50
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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