Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	$ 9,433
Due after one year through five years - amortized cost	7,225
Due after five years through ten years - amortized cost	487
Due after ten years - amortized cost	206
Amortized cost of fixed maturity securities	17,602	$ 19,724
Due in one year or less - fair value	9,436
Due after one year through five years - fair value	7,283
Due after five years through ten years - fair value	758
Due after ten years - fair value	350
Fair value of fixed maturity securities	18,125	$ 20,410
Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Amortized cost of mortgage-backed securities	251
Fair value of mortgage-backed securities	$ 298

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
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-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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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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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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