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Claim and Claim Adjustment Expense Reserves (Specialty - Surety - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Specialty - USD ($) $ in Millions | 12 Months Ended | 24 Months Ended | 36 Months Ended | 48 Months Ended | 60 Months Ended | 72 Months Ended | 84 Months Ended | 96 Months Ended | 108 Months Ended | 120 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2022 | Dec. 31, 2023 | Dec. 31, 2023 | Dec. 31, 2023 | Dec. 31, 2023 | Dec. 31, 2023 | Dec. 31, 2023 | Dec. 31, 2023 | Dec. 31, 2023
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total | $ (68) | $ (43) | $ (83) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Surety |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (63) | (54) | (82) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total net development for accident years prior to 2015 | (5) | 11 | (1) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total unallocated claim adjustment expense development | 0 | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total | (68) | (43) | (83) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Surety | 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 0 | 0 | (8) | $ (5) | $ (5) | $ (16) | $ (25) | $ (27) | $ 0 |  |  |  |  |  |  |  |  | $ (86)
Surety | 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 0 | (15) | (6) | (3) | (17) | (25) | (15) | $ 0 |  |  |  |  |  |  |  |  | $ (81) |
Surety | 2017 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 0 | 1 | (5) | (13) | (19) | (12) | $ (5) |  |  |  |  |  |  |  |  | $ (53) |  |
Surety | 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (2) | (5) | (6) | (29) | (17) | $ (6) |  |  |  |  |  |  |  |  | $ (65) |  |  |
Surety | 2019 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 0 | (5) | (11) | (14) | $ (7) |  |  |  |  |  |  |  |  | $ (37) |  |  |  |
Surety | 2020 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (10) | (14) | (38) | $ (9) |  |  |  |  |  |  |  |  | $ (71) |  |  |  |  |
Surety | 2021 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (19) | (19) | $ (8) |  |  |  |  |  |  |  |  | $ (46) |  |  |  |  |  |
Surety | 2022 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (26) | $ 3 |  |  |  |  |  |  |  |  | $ (23) |  |  |  |  |  |  |
Surety | 2023 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | $ (6) |  |  |  |  |  |  |  |  | $ (6) |  |  |  |  |  |  |  |

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