Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	$ 7,198
Due after one year through five years - amortized cost	10,331
Due after five years through ten years - amortized cost	364
Due after ten years - amortized cost	459
Amortized cost of fixed maturity securities	18,822	$ 19,427
Due in one year or less - fair value	7,242
Due after one year through five years - fair value	10,443
Due after five years through ten years - fair value	410
Due after ten years - fair value	789
Fair value of fixed maturity securities	19,415	19,898
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of mortgage-backed securities	470
Amortized cost of fixed maturity securities	470	487
Fair value of mortgage-backed securities	531
Fair value of fixed maturity securities	$ 531	$ 544

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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), with single maturity date and allocated without single maturity date, maturing in rolling sixth through tenth fiscal year following latest fiscal year. For interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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