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Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred Loss and Allocated LAE, Net of Reinsurance - Specialty Industries - Casualty) (Details) - Casualty - Specialty Industries $ in Millions | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2011 USD ($) | Dec. 31, 2010 USD ($) | Dec. 31, 2009 USD ($) | Dec. 31, 2008 USD ($) | Dec. 31, 2007 USD ($)
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Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 918.2 |  |  |  |  |  |  |  |  |
2007 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 1,770 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 36.9 | $ 37.1 | $ 36.0 | $ 35.1 | $ 35.1 | $ 34.9 | $ 34.0 | $ 21.7 | $ 24.3 | $ 24.9
Total IBNR plus expected development on reported claims | $ 0.3 |  |  |  |  |  |  |  |  |
2008 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 1,835 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 22.1 | 22.2 | 23.7 | 25.2 | 25.8 | 26.4 | 30.4 | 26.2 | $ 26.3 |
Total IBNR plus expected development on reported claims | $ 0.5 |  |  |  |  |  |  |  |  |
2009 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 2,982 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 51.0 | 51.6 | 53.6 | 54.8 | 49.6 | 51.5 | 51.1 | $ 51.0 |  |
Total IBNR plus expected development on reported claims | $ 1.0 |  |  |  |  |  |  |  |  |
2010 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 4,791 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 77.2 | 75.5 | 73.3 | 69.4 | 69.3 | 72.4 | $ 80.8 |  |  |
Total IBNR plus expected development on reported claims | $ 3.5 |  |  |  |  |  |  |  |  |
2011 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 5,254 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 112.2 | 111.8 | 100.5 | 90.2 | 87.5 | $ 88.1 |  |  |  |
Total IBNR plus expected development on reported claims | $ 3.8 |  |  |  |  |  |  |  |  |
2012 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 6,298 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 98.8 | 101.8 | 102.0 | 103.4 | $ 118.8 |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 7.9 |  |  |  |  |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 7,107 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 138.2 | 133.6 | 131.5 | $ 121.9 |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 18.0 |  |  |  |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 7,633 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 124.2 | 127.4 | $ 135.6 |  |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 35.7 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 7,315 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 127.7 | $ 146.3 |  |  |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 61.8 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | 5,701 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 129.9 |  |  |  |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 98.4 |  |  |  |  |  |  |  |  |

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