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Reserve for Losses and Loss Adjustment Expenses - Prior year development (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2025 | Mar. 31, 2024
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | $ (197) | $ (124)
Operating segments |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | (197) | (124)
Operating segments | Short tailed lines |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | (203) | (133)
Operating segments | Long tailed lines |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | 6 | 9
Operating segments | Insurance |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | (17) | (10)
Operating segments | Insurance | Short tailed lines |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | (15) | (17)
Operating segments | Insurance | Short tailed lines | Property And Marine Product Line |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | (8) |
Operating segments | Insurance | Short tailed lines | Travel and accident |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | (8) | (9)
Operating segments | Insurance | Long tailed lines |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | (2) | 7
Operating segments | Insurance | Long tailed lines | Programs business |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse |  | 8
Operating segments | Reinsurance |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | (119) | (40)
Operating segments | Reinsurance | Short tailed lines |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | (127) | (42)
Operating segments | Reinsurance | Short tailed lines | Property catastrophe |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | (64) |
Operating segments | Reinsurance | Short tailed lines | Other specialty lines |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | (35) | (33)
Operating segments | Reinsurance | Long tailed lines |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | 8 | 2
Operating segments | Reinsurance | Long tailed lines | Casualty |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | 8 |
Operating segments | Mortgage |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | (61) | (74)
Operating segments | Mortgage | Short tailed lines |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | (61) | (74)
Operating segments | Mortgage | Long tailed lines |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |
Prior year development (favorable) adverse | $ 0 | $ 0

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- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-7 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 + Details
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