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Unpaid losses and loss expenses Unpaid losses and loss expenses (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2024 | Jun. 30, 2023 | Jun. 30, 2024 | Jun. 30, 2023
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease) |  |  | $ 2,544 |
Net Prior Period Development | $ (192) | $ (200) | (399) | $ (396)
Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (8) | (72) | 96 | (53)
Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (184) | (128) | (495) | (343)
North America Commercial P&C Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (144) | (146) | (192) | (218)
North America Commercial P&C Insurance [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (142) | (139) | (46) | (130)
North America Commercial P&C Insurance [Member] | Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (2) | (7) | (146) | (88)
North America Commercial P&C Insurance [Member] | Workers' Compensation Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (288) | (264) | (338) | (308)
North America Commercial P&C Insurance [Member] | Auto Liability [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 117 | 119 | 167 | 125
North America Commercial P&C Insurance [Member] | Surety, Property and Marine Lines [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  |  | (144) |
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 |  |  | 97 |
Overseas General Insurance [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (61) | (61) | (150) | (204)
Overseas General Insurance [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 36 | 0 | 35 | 0
Overseas General Insurance [Member] | Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | (97) | (61) | (185) | (204)
Overseas General Insurance [Member] | Segment Insurance Overseas General, Property |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  |  | (85) |
Overseas General Insurance [Member] | Segment Insurance Overseas General, Marine |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development |  |  | (73) |
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 |  | 110 |  |
Corporate Segment [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 93 | 60 | 102 | 70
Corporate Segment [Member] | Long Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | 93 | 60 | 102 | 70
Corporate Segment [Member] | Short Tail [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Net Prior Period Development | $ 0 | $ 0 | $ 0 | $ 0

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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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