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Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2024 | Mar. 31, 2023
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease) | $ 940 |
Net Prior Period Development | (207) | $ (196)
Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 104 | 19
Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (311) | (215)
North America Commercial P&C Insurance [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (48) | (72)
North America Commercial P&C Insurance [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 96 | 9
North America Commercial P&C Insurance [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (144) | (81)
North America Commercial P&C Insurance [Member] | Commercial Excess and Umbrella [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | 102 |
North America Commercial P&C Insurance [Member] | Surety Product Line [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (74) |
North America Commercial P&C Insurance [Member] | Property and Inland Marine |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (62) |
Overseas General Insurance [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (89) | (143)
Overseas General Insurance [Member] | Long Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (1) | 0
Overseas General Insurance [Member] | Short Tail [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | (88) | (143)
Overseas General Insurance [Member] | Property, Liability and Casualty Insurance Product Line |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development | $ (58) | (85)
Overseas General Insurance [Member] | Property, Liability and Casualty Insurance Product Line | Catastrophe [Member] |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Net Prior Period Development |  | $ (43)

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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://fasb.org/us-gaap/role/ref/legacyRef -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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