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Retroactive reinsurance contracts - Narrative (Detail) - Retroactive Reinsurance [Member] - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Effects of Reinsurance [Line Items] |  |  |
Insurance losses and loss adjustment expenses | $ 945 | $ 1,484 | $ 855
Increase in incurred losses and loss adjustment expenses |  | 650 |
Deferred charge assets on retroactive reinsurance contracts | 8,800 | 9,500 |
Liabilities for environmental, asbestos and latent injury claims | 11,900 | 12,200 |
Unpaid Losses and Loss Adjustment Expenses [Member] |  |  |
Effects of Reinsurance [Line Items] |  |  |
Insurance losses and loss adjustment expenses | $ 247 | $ 1,109 | $ 86

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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://www.xbrl.org/2009/role/commonPracticeRef -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://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -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 increase (decrease) in the liability for unpaid claims and claims adjustment expense. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 + Details
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