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Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease) |  |  | $ 3,748 |
Net Prior Period Development | $ (361) | $ (244) | (865) | $ (643)
Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 99 | 114 | 186 | 210
Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (460) | (358) | (1,051) | (853)
North America Commercial P&C Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (26) | (39) | (246) | (231)
North America Commercial P&C Insurance [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 104 | 125 | 77 | 79
North America Commercial P&C Insurance [Member] | Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (130) | (164) | (323) | (310)
Overseas General Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (84) | (60) | (282) | (210)
Overseas General Insurance [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (66) | (61) | (30) | (26)
Overseas General Insurance [Member] | Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | $ (18) | $ 1 | $ (252) | $ (184)

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- Definition Net Prior Period Development - Amount, after effects of reinstatement premiums, expense adjustments, and net premiums earned adjustments for claims incurred in prior reporting periods and related claims settlement costs. + References No definition available. + Details
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- Definition 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. + References No definition available. + 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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