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Claim and Claim Adjustment Expense Reserves (Commercial - Workers' Compensation - 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
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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, Workers' Compensation | $ (65) | $ 150 | $ 80 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Workers' Compensation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (76) | 106 | 71 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Adjustment for development on a discounted basis | (4) | 1 | (2) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total net development for accident years prior to 2008 | 14 | 43 | 11 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total unallocated claim adjustment expense development | 1 | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net prior year claim and allocated claim adjustment expense reserve development, Workers' Compensation | (65) | 150 | 80 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Workers' Compensation | 2008 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 14 | 8 | 8 | $ 10 | $ 4 | $ 2 | $ 13 | $ 18 | $ 17 |  |  |  |  |  |  |  | $ 94
Workers' Compensation | 2009 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 7 | 8 | 6 | 11 | 4 | 2 | 7 | $ 4 |  |  |  |  |  |  |  | $ 49 |
Workers' Compensation | 2010 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 4 | 20 | 14 | 20 | 22 | 22 | $ 43 |  |  |  |  |  |  |  | $ 145 |  |
Workers' Compensation | 2011 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 2 | 24 | (6) | 11 | 9 | $ 35 |  |  |  |  |  |  |  | $ 75 |  |  |
Workers' Compensation | 2012 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (4) | 10 | 13 | 32 | $ 27 |  |  |  |  |  |  |  | $ 78 |  |  |  |
Workers' Compensation | 2013 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (26) | 26 | 21 | $ 35 |  |  |  |  |  |  |  | $ 56 |  |  |  |  |
Workers' Compensation | 2014 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (26) | 0 | $ 15 |  |  |  |  |  |  |  | $ (11) |  |  |  |  |  |
Workers' Compensation | 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (25) | 10 |  |  |  |  |  |  |  | $ (15) |  |  |  |  |  |  |
Workers' Compensation | 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | $ (22) | $ (22) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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