Investments in fixed maturity securities (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	$ 19,276	$ 20,445
Unrealized gains on fixed maturity securities	731	977
Unrealized losses on fixed maturity securities	(87)	(69)
Fair value of fixed maturity securities	19,920	21,353
U.S. Treasury, U.S. government corporations and agencies [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	3,633	3,975
Unrealized gains on fixed maturity securities	11	4
Unrealized losses on fixed maturity securities	(36)	(26)
Fair value of fixed maturity securities	3,608	3,953
States, municipalities and political subdivisions [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	737	847
Unrealized gains on fixed maturity securities	30	19
Unrealized losses on fixed maturity securities	(8)	(12)
Fair value of fixed maturity securities	759	854
Foreign governments [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	7,837	8,572
Unrealized gains on fixed maturity securities	83	274
Unrealized losses on fixed maturity securities	(32)	(24)
Fair value of fixed maturity securities	7,888	8,822
Corporate bonds [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	6,308	6,279
Unrealized gains on fixed maturity securities	519	588
Unrealized losses on fixed maturity securities	(8)	(5)
Fair value of fixed maturity securities	6,819	6,862
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	761	772
Unrealized gains on fixed maturity securities	88	92
Unrealized losses on fixed maturity securities	(3)	(2)
Fair value of fixed maturity securities	$ 846	$ 862

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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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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