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Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, current accident year | $ 59,244,000,000 | $ 59,463,000,000 | $ 52,099,000,000
Incurred losses and loss adjustment expenses, prior accident years | $ (3,541,000,000) | $ (2,672,000,000) | $ (3,116,000,000)
Prior years' losses incurred as a percentage of net liabilities | 3.50% | 3.20% | 4.00%
Liabilities for environmental, asbestos and latent injury exposures | $ 2,000,000,000 | $ 2,100,000,000 |
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | Catastrophe [Member] |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, current accident year | 925,000,000 | 3,100,000,000 | $ 2,900,000,000
Minimum losses per event each accident year | 150,000,000 |  |
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | Hurricane Ian [Member] |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, current accident year |  | 2,500,000,000 |
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | Hurricane Ida [Member] |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, current accident year |  |  | 1,500,000,000
Property and Casualty Reinsurance, Excluding Retroactive Reinsurance [Member] | Primary Insurance Businesses [Member] |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | (2,100,000,000) | (1,100,000,000) | (2,400,000,000)
Property and Casualty Reinsurance, Excluding Retroactive Reinsurance [Member] | Property And Casualty Reinsurance [Member] |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Incurred losses and loss adjustment expenses, prior accident years | $ (1,400,000,000) | $ (1,600,000,000) | $ (718,000,000)

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- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, net of estimated reinsurance recoveries. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (cc) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 + Details
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- Definition Amount, after effects of reinsurance, of expense for claims incurred in the current reporting period and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 + Details
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