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Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Commercial - General Liability - 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, 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
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total | $ 15 | $ 15 | $ 16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 14 | 16 | 21 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total net development for accident years prior to 2012 | (1) | (1) | (4) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total unallocated claim adjustment expense development | 2 | 0 | (1) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total | 15 | 15 | 16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Liability | 2012 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 0 | 2 | (5) | $ 0 | $ 16 | $ (17) | $ (3) | $ 28 | $ 24 |  |  |  |  |  |  |  |  | $ 45
General Liability | 2013 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 1 | 3 | (3) | 10 | (42) | 5 | (5) | $ 5 |  |  |  |  |  |  |  |  | $ (26) |
General Liability | 2014 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 0 | 1 | 23 | 4 | (23) | (4) | $ 5 |  |  |  |  |  |  |  |  | $ 6 |  |
General Liability | 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 15 | 2 | 11 | 15 | (2) | $ (5) |  |  |  |  |  |  |  |  | $ 36 |  |  |
General Liability | 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 10 | 2 | 4 | 8 | $ 36 |  |  |  |  |  |  |  |  | $ 60 |  |  |  |
General Liability | 2017 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (4) | 2 | 0 | $ 0 |  |  |  |  |  |  |  |  | $ (2) |  |  |  |  |
General Liability | 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (7) | 2 | $ (9) |  |  |  |  |  |  |  |  | $ (14) |  |  |  |  |  |
General Liability | 2019 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 0 | $ 2 |  |  |  |  |  |  |  |  | $ 2 |  |  |  |  |  |  |
General Liability | 2020 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | $ (1) |  |  |  |  |  |  |  |  | $ (1) |  |  |  |  |  |  |  |

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