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Claim and Claim Adjustment Expense Reserves and Future Policy Benefit Reserves (Specialty - Other Professional Liability and Management Liability - 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, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2022 | Dec. 31, 2022 | Dec. 31, 2022 | Dec. 31, 2022 | Dec. 31, 2022 | Dec. 31, 2022 | Dec. 31, 2022 | Dec. 31, 2022 | Dec. 31, 2022
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total | $ 50 | $ 24 | $ (15) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Professional Liability and Management Liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 41 | 36 | (13) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total net development for accident years prior to 2013 | 9 | (14) | (2) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total unallocated claim adjustment expense development | 0 | 2 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total | 50 | 24 | (15) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Professional Liability and Management Liability | 2013 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (4) | (13) | (4) | $ (13) | $ (3) | $ (19) | $ (41) | $ 32 | $ 10 |  |  |  |  |  |  |  |  | $ (55)
Other Professional Liability and Management Liability | 2014 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 1 | (4) | (9) | 19 | 4 | (54) | (13) | $ 20 |  |  |  |  |  |  |  |  | $ (36) |
Other Professional Liability and Management Liability | 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 19 | 23 | 6 | (25) | (45) | (15) | $ 4 |  |  |  |  |  |  |  |  | $ (33) |  |
Other Professional Liability and Management Liability | 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (3) | (16) | 3 | 4 | 0 | $ (1) |  |  |  |  |  |  |  |  | $ (13) |  |  |
Other Professional Liability and Management Liability | 2017 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (17) | (16) | (22) | (32) | $ (2) |  |  |  |  |  |  |  |  | $ (89) |  |  |  |
Other Professional Liability and Management Liability | 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 17 | 37 | 5 | $ 14 |  |  |  |  |  |  |  |  | $ 73 |  |  |  |  |
Other Professional Liability and Management Liability | 2019 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 20 | 11 | $ 8 |  |  |  |  |  |  |  |  | $ 39 |  |  |  |  |  |
Other Professional Liability and Management Liability | 2020 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 7 | $ 14 |  |  |  |  |  |  |  |  | $ 21 |  |  |  |  |  |  |
Other Professional Liability and Management Liability | 2021 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | $ 1 |  |  |  |  |  |  |  |  | $ 1 |  |  |  |  |  |  |  |

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