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Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Specialty - Medical Professional 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 | $ 23 | $ 35 | $ 75 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Medical Professional Liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 13 | 32 | 52 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total net development for accident years prior to 2012 | 3 | 3 | 21 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total unallocated claim adjustment expense development | 7 | 0 | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total | 23 | 35 | 75 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Medical Professional Liability | 2012 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 0 | (2) | 6 | $ 9 | $ (9) | $ (5) | $ (10) | $ 39 | $ 5 |  |  |  |  |  |  |  |  | $ 33
Medical Professional Liability | 2013 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (1) | (14) | 10 | 10 | 12 | 13 | 21 | $ 17 |  |  |  |  |  |  |  |  | $ 68 |
Medical Professional Liability | 2014 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (3) | (2) | (6) | 5 | (7) | 48 | $ 39 |  |  |  |  |  |  |  |  | $ 74 |  |
Medical Professional Liability | 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (9) | 22 | (6) | (16) | 11 | $ 66 |  |  |  |  |  |  |  |  | $ 68 |  |  |
Medical Professional Liability | 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 2 | 9 | 14 | (2) | $ 60 |  |  |  |  |  |  |  |  | $ 83 |  |  |  |
Medical Professional Liability | 2017 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (5) | 2 | 9 | $ 37 |  |  |  |  |  |  |  |  | $ 43 |  |  |  |  |
Medical Professional Liability | 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 17 | 2 | $ 25 |  |  |  |  |  |  |  |  | $ 44 |  |  |  |  |  |
Medical Professional Liability | 2019 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 13 | $ 15 |  |  |  |  |  |  |  |  | $ 28 |  |  |  |  |  |  |
Medical Professional Liability | 2020 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | $ (1) |  |  |  |  |  |  |  |  | $ (1) |  |  |  |  |  |  |  |

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