Investments in fixed maturity securities (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	$ 19,427	$ 20,445
Unrealized gains on fixed maturity securities	546	977
Unrealized losses on fixed maturity securities	(75)	(69)
Fair value of fixed maturity securities	19,898	21,353
U.S. Treasury, U.S. government corporations and agencies [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	4,223	3,975
Unrealized gains on fixed maturity securities	22	4
Unrealized losses on fixed maturity securities	(22)	(26)
Fair value of fixed maturity securities	4,223	3,953
States, municipalities and political subdivisions [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	182	847
Unrealized gains on fixed maturity securities	7	19
Unrealized losses on fixed maturity securities		(12)
Fair value of fixed maturity securities	189	854
Foreign governments [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	7,480	8,572
Unrealized gains on fixed maturity securities	50	274
Unrealized losses on fixed maturity securities	(28)	(24)
Fair value of fixed maturity securities	7,502	8,822
Corporate bonds [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	7,055	6,279
Unrealized gains on fixed maturity securities	408	588
Unrealized losses on fixed maturity securities	(23)	(5)
Fair value of fixed maturity securities	7,440	6,862
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	487	772
Unrealized gains on fixed maturity securities	59	92
Unrealized losses on fixed maturity securities	(2)	(2)
Fair value of fixed maturity securities	$ 544	$ 862

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Section 50
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
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