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Claim, Claim Adjustment Expense 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, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2021 | Dec. 31, 2021 | Dec. 31, 2021 | Dec. 31, 2021 | Dec. 31, 2021 | Dec. 31, 2021 | Dec. 31, 2021 | Dec. 31, 2021 | Dec. 31, 2021
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total | $ 24 | $ (15) | $ (69) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Professional Liability and Management Liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 34 | 6 | (35) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total net development for accident years prior to 2012 | (12) | (21) | (20) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total unallocated claim adjustment expense development | 2 | 0 | (14) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total | 24 | (15) | (69) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Professional Liability and Management Liability | 2012 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (2) | 19 | (2) | $ (13) | $ 6 | $ (38) | $ (9) | $ (22) | $ (14) |  |  |  |  |  |  |  |  | $ (75)
Other Professional Liability and Management Liability | 2013 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (13) | (4) | (13) | (3) | (19) | (41) | 32 | $ 10 |  |  |  |  |  |  |  |  | $ (51) |
Other Professional Liability and Management Liability | 2014 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (4) | (9) | 19 | 4 | (54) | (13) | $ 20 |  |  |  |  |  |  |  |  | $ (37) |  |
Other Professional Liability and Management Liability | 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 23 | 6 | (25) | (45) | (15) | $ 4 |  |  |  |  |  |  |  |  | $ (52) |  |  |
Other Professional Liability and Management Liability | 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (16) | 3 | 4 | 0 | $ (1) |  |  |  |  |  |  |  |  | $ (10) |  |  |  |
Other Professional Liability and Management Liability | 2017 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (16) | (22) | (32) | $ (2) |  |  |  |  |  |  |  |  | $ (72) |  |  |  |  |
Other Professional Liability and Management Liability | 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 37 | 5 | $ 14 |  |  |  |  |  |  |  |  | $ 56 |  |  |  |  |  |
Other Professional Liability and Management Liability | 2019 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 11 | $ 8 |  |  |  |  |  |  |  |  | $ 19 |  |  |  |  |  |  |
Other Professional Liability and Management Liability | 2020 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | $ 14 |  |  |  |  |  |  |  |  | $ 14 |  |  |  |  |  |  |  |

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