Investments in fixed maturity securities (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	$ 21,273	$ 22,434
Unrealized gains on fixed maturity securities	1,026	1,121
Unrealized losses on fixed maturity securities	(43)	(90)
Fair value of fixed maturity securities	22,256	23,465
U.S. Treasury, U.S. government corporations and agencies [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	4,327	4,519
Unrealized gains on fixed maturity securities	5	16
Unrealized losses on fixed maturity securities	(11)	(8)
Fair value of fixed maturity securities	4,321	4,527
States, municipalities and political subdivisions [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	988	1,159
Unrealized gains on fixed maturity securities	51	58
Unrealized losses on fixed maturity securities	(1)	(1)
Fair value of fixed maturity securities	1,038	1,216
Foreign governments [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	8,556	8,860
Unrealized gains on fixed maturity securities	233	207
Unrealized losses on fixed maturity securities	(24)	(66)
Fair value of fixed maturity securities	8,765	9,001
Corporate bonds [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	6,525	6,899
Unrealized gains on fixed maturity securities	628	714
Unrealized losses on fixed maturity securities	(4)	(9)
Fair value of fixed maturity securities	7,149	7,604
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	877	997
Unrealized gains on fixed maturity securities	109	126
Unrealized losses on fixed maturity securities	(3)	(6)
Fair value of fixed maturity securities	$ 983	$ 1,117

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