Retroactive reinsurance contracts - Narrative (Detail) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Effects Of Reinsurance [Line Items]
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts	$ 39,721	$ 41,786	$ 40,966	$ 42,441
Deferred charges under retroactive reinsurance contracts	11,732	13,012	12,441	$ 13,747
Retroactive Reinsurance [Member]
Effects Of Reinsurance [Line Items]
Incurred losses and loss adjustment expenses for (reductions of) prior years' accident events	700	$ 866
NICO [Member] \| AIG [Member] \| Retroactive Reinsurance [Member]
Effects Of Reinsurance [Line Items]
Maximum contractual liability under contract	20,000
Unpaid losses and loss adjustment expenses under retroactive reinsurance contracts	16,800		17,700
Retroactive reinsurance contracts, claims paid	837
Deferred charges under retroactive reinsurance contracts	$ 5,000		$ 5,400

Unpaid losses and loss adjustment expenses for retroactive reinsurance contracts.

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- Definition

Maximum contractual liability under contract.

+ References

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- Definition

The remaining unamortized excess of the estimated ultimate liability for claims and claim costs over the premiums earned with respect to retroactive property/casualty reinsurance contracts.

+ References

No definition available.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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- Definition

Amount, after effects of reinsurance, of payments to settle insured claims and pay costs incurred in the claims settlement process.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3536-108585

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- Definition

Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

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