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Claim and Claim Adjustment Expense Reserves (Commercial - Commercial Auto - 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
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto | $ (38) | $ (46) | $ (22) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial Auto |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (41) | (40) | (18) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total net development for accident years prior to 2008 | 1 | (6) | (4) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total unallocated claim adjustment expense development | 2 | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto | (38) | (46) | (22) |  |  |  |  |  |  |  |  |  |  |  |  |  |
2008 | Commercial Auto |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 1 | (2) | 0 | $ (7) | $ (4) | $ 3 | $ (10) | $ (7) | $ 1 |  |  |  |  |  |  |  | $ (25)
2009 | Commercial Auto |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 0 | (3) | (2) | (4) | 3 | 4 | 2 | $ (15) |  |  |  |  |  |  |  | $ (15) |
2010 | Commercial Auto |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (1) | (5) | (5) | 8 | 4 | 5 | $ 12 |  |  |  |  |  |  |  | $ 18 |  |
2011 | Commercial Auto |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 0 | (5) | (3) | 14 | 6 | $ 11 |  |  |  |  |  |  |  | $ 23 |  |  |
2012 | Commercial Auto |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (8) | 4 | 4 | 10 | $ 12 |  |  |  |  |  |  |  | $ 22 |  |  |  |
2013 | Commercial Auto |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (4) | (16) | (2) | $ 17 |  |  |  |  |  |  |  | $ (5) |  |  |  |  |
2014 | Commercial Auto |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (6) | (11) | $ (10) |  |  |  |  |  |  |  | $ (27) |  |  |  |  |  |
2015 | Commercial Auto |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (10) | (2) |  |  |  |  |  |  |  | $ (12) |  |  |  |  |  |  |
2016 | Commercial Auto |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | $ (13) | $ (13) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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