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Reserve for Losses and Loss Adjustment Expenses - Prior year development (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior year development (favorable) adverse | $ (153) | $ (121) | $ (350) | $ (245)
Operating segments |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior year development (favorable) adverse | (153) | (121) | (350) | (245)
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 | (152) | (143) | (355) | (277)
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 | (1) | 22 | 5 | 32
Operating segments | Insurance |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior year development (favorable) adverse | (8) | (5) | (25) | (15)
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 | (13) | (13) | (28) | (30)
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 | (7) |  | (15) |
Operating segments | Insurance | Short tailed lines | Property, energy, marine and aviation |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior year development (favorable) adverse |  | 15 | (11) |
Operating segments | Insurance | Short tailed lines | Surety Product Line |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior year development (favorable) adverse |  | (20) |  | (25)
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 | 5 | 8 | 3 | 15
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 | 14 | 9 |  | 17
Operating segments | Reinsurance |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior year development (favorable) adverse | (81) | (34) | (200) | (74)
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 | (75) | (48) | (202) | (91)
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 |  |  | (86) |
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 |  |  |  | (37)
Operating segments | Reinsurance | Short tailed lines | Property other than property catastrophe |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior year development (favorable) adverse | (60) | (30) | (89) | (51)
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 | (6) | 14 | 2 | 17
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 |  | 14 |  | 17
Operating segments | Mortgage |  |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior year development (favorable) adverse | (64) | (82) | (125) | (156)
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 | (64) | (82) | (125) | (156)
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 | $ 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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