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Claim and Claim Adjustment Expense Reserves (Specialty - Medical Professional 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
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
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] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total | $ 5 | $ (37) | $ (43) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Medical Professional Liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 32 | (2) | (27) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total net development for accident years prior to 2008 | (19) | (35) | (16) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total unallocated claim adjustment expense development | (8) | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total | 5 | (37) | (43) |  |  |  |  |  |  |  |  |  |  |  |  |  |
Medical Professional Liability | 2008 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (4) | (13) | (12) | $ (1) | $ 0 | $ (12) | $ (16) | $ 45 | $ 25 |  |  |  |  |  |  |  | $ 12
Medical Professional Liability | 2009 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (10) | (31) | (8) | (9) | (26) | 12 | 25 | $ 7 |  |  |  |  |  |  |  | $ (40) |
Medical Professional Liability | 2010 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (5) | (43) | (24) | (16) | 8 | 0 | $ (5) |  |  |  |  |  |  |  | $ (85) |  |
Medical Professional Liability | 2011 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 0 | (10) | (32) | 26 | 15 | $ 6 |  |  |  |  |  |  |  | $ 5 |  |  |
Medical Professional Liability | 2012 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 4 | (8) | (8) | 46 | $ 3 |  |  |  |  |  |  |  | $ 37 |  |  |  |
Medical Professional Liability | 2013 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 6 | 7 | 20 | $ 6 |  |  |  |  |  |  |  | $ 39 |  |  |  |  |
Medical Professional Liability | 2014 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | (21) | 37 | $ 37 |  |  |  |  |  |  |  | $ 53 |  |  |  |  |  |
Medical Professional Liability | 2015 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | 4 | 59 |  |  |  |  |  |  |  | $ 63 |  |  |  |  |  |  |
Medical Professional Liability | 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Number of Claims | $ 58 | $ 58 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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