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Claim and Claim Adjustment Expense Reserves - Net Prior Year Loss Reserve Development in Property and Casualty Operations (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Total pretax (favorable) unfavorable development |  |  |  |
Net Prior Year Development [Abstract] |  |  |  |
Total pretax (favorable) unfavorable development | $ (1) | $ 17 | $ 190 | $ 33
Property and casualty operations |  |  |  |
Net Prior Year Development [Abstract] |  |  |  |
Total pretax (favorable) unfavorable development | (1) | (5) | 56 | (24)
Medical professional liability |  |  |  |
Net Prior Year Development [Abstract] |  |  |  |
Total pretax (favorable) unfavorable development |  |  |  | (2)
Other professional liability and management liability |  |  |  |
Net Prior Year Development [Abstract] |  |  |  |
Total pretax (favorable) unfavorable development | 25 | 11 | 43 | 28
Surety |  |  |  |
Net Prior Year Development [Abstract] |  |  |  |
Total pretax (favorable) unfavorable development | (25) | (20) | (47) | (46)
Warranty |  |  |  |
Net Prior Year Development [Abstract] |  |  |  |
Total pretax (favorable) unfavorable development |  | 7 | 10 | 20
Commercial auto |  |  |  |
Net Prior Year Development [Abstract] |  |  |  |
Total pretax (favorable) unfavorable development |  | 25 | 50 | 46
General liability |  |  |  |
Net Prior Year Development [Abstract] |  |  |  |
Total pretax (favorable) unfavorable development |  | 28 | 62 | 47
Workers’ compensation |  |  |  |
Net Prior Year Development [Abstract] |  |  |  |
Total pretax (favorable) unfavorable development | (1) | (57) | (66) | (106)
Other property and casualty operations |  |  |  |
Net Prior Year Development [Abstract] |  |  |  |
Total pretax (favorable) unfavorable development |  | 1 | 4 | (11)
Other insurance operations |  |  |  |
Net Prior Year Development [Abstract] |  |  |  |
Total pretax (favorable) unfavorable development |  | $ 22 | $ 134 | $ 57

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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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