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Claim and Claim Adjustment Expense Reserves (Specialty - Other Professional Liability and Management 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
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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
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management Liability | $ (131) | $ (130) | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Professional Liability and Management Liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (112) | (134) | 26 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total net development for accident years prior to 2008 | (14) | 4 | (26) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total unallocated claim adjustment expense development | (5) | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management Liability | (131) | (130) | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Professional Liability and Management Liability | 2008 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (9) | (18) | (5) | $ (30) | $ (35) | $ (9) | $ (30) | $ 21 | $ 17 |  |  |  |  |  |  |  | $ (98)
Other Professional Liability and Management Liability | 2009 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (2) | 8 | (5) | 9 | (7) | (5) | 30 | $ 44 |  |  |  |  |  |  |  | $ 72 |
Other Professional Liability and Management Liability | 2010 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 3 | (13) | (10) | (2) | (2) | 23 | $ 2 |  |  |  |  |  |  |  | $ 1 |  |
Other Professional Liability and Management Liability | 2011 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (12) | (34) | (4) | 15 | 29 | $ 28 |  |  |  |  |  |  |  | $ 22 |  |  |
Other Professional Liability and Management Liability | 2012 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (1) | (37) | (6) | (18) | $ (13) |  |  |  |  |  |  |  | $ (75) |  |  |  |
Other Professional Liability and Management Liability | 2013 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (16) | (39) | 38 | $ (3) |  |  |  |  |  |  |  | $ (20) |  |  |  |  |
Other Professional Liability and Management Liability | 2014 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (56) | (5) | $ 18 |  |  |  |  |  |  |  | $ (43) |  |  |  |  |  |
Other Professional Liability and Management Liability | 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (19) | 4 |  |  |  |  |  |  |  | $ (15) |  |  |  |  |  |  |
Other Professional Liability and Management Liability | 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | $ 0 | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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