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Claim and Claim Adjustment Expense 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
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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, General Liability | $ 0 | $ (28) | $ (33) |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (5) | (13) | (49) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total net development for accident years prior to 2008 | (2) | (15) | 16 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total unallocated claim adjustment expense development | 7 | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net prior year claim and allocated claim adjustment expense reserve development, General Liability | 0 | (28) | (33) |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Liability | 2008 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 1 | (9) | (13) | $ (19) | $ (1) | $ (14) | $ 17 | $ 26 | $ (7) |  |  |  |  |  |  |  | $ (19)
General Liability | 2009 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 5 | 3 | (4) | (6) | (4) | (1) | (3) | $ 46 |  |  |  |  |  |  |  | $ 36 |
General Liability | 2010 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (5) | (16) | (20) | 46 | 50 | 2 | $ 31 |  |  |  |  |  |  |  | $ 88 |  |
General Liability | 2011 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (6) | 10 | 1 | 46 | 30 | $ (3) |  |  |  |  |  |  |  | $ 78 |  |  |
General Liability | 2012 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 11 | (12) | (9) | 16 | $ 24 |  |  |  |  |  |  |  | $ 30 |  |  |  |
General Liability | 2013 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (39) | 9 | (11) | $ 30 |  |  |  |  |  |  |  | $ (11) |  |  |  |  |
General Liability | 2014 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (8) | 1 | $ 7 |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |
General Liability | 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 11 | 1 |  |  |  |  |  |  |  | $ 12 |  |  |  |  |  |  |
General Liability | 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | $ 25 | $ 25 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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