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Claim and Claim Adjustment Expense Reserves (Commercial - Workers' Compensation - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Commercial - 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, 2023 | Dec. 31, 2023 | Dec. 31, 2023 | Dec. 31, 2023 | Dec. 31, 2023 | Dec. 31, 2023 | Dec. 31, 2023 | Dec. 31, 2023 | Dec. 31, 2023
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total | $ (202) | $ (203) | $ (152) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Workers' Compensation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (95) | (133) | (81) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustment for development on a discounted basis | (2) | (2) | (3) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total net development for accident years prior to 2015 | (105) | (74) | (78) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total unallocated claim adjustment expense development | 0 | 6 | 10 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total | (202) | (203) | (152) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Workers' Compensation | 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (10) | (19) | (19) | $ (10) | $ (12) | $ (14) | $ 2 | $ (25) | $ 9 |  |  |  |  |  |  |  |  | $ (98)
Workers' Compensation | 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (15) | (23) | (24) | (11) | (16) | (14) | (9) | $ (21) |  |  |  |  |  |  |  |  | $ (133) |
Workers' Compensation | 2017 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (15) | (1) | (3) | 2 | (21) | (11) | $ (8) |  |  |  |  |  |  |  |  | $ (57) |  |
Workers' Compensation | 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (5) | (11) | 0 | (13) | (12) | $ (10) |  |  |  |  |  |  |  |  | $ (51) |  |  |
Workers' Compensation | 2019 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (3) | (17) | (1) | (12) | $ (3) |  |  |  |  |  |  |  |  | $ (36) |  |  |  |
Workers' Compensation | 2020 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (21) | (32) | (20) | $ (11) |  |  |  |  |  |  |  |  | $ (84) |  |  |  |  |
Workers' Compensation | 2021 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (11) | (22) | $ (14) |  |  |  |  |  |  |  |  | $ (47) |  |  |  |  |  |
Workers' Compensation | 2022 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (11) | $ (8) |  |  |  |  |  |  |  |  | $ (19) |  |  |  |  |  |  |
Workers' Compensation | 2023 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | $ (4) |  |  |  |  |  |  |  |  | $ (4) |  |  |  |  |  |  |  |

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