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Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred Loss and Allocated LAE, Net of Reinsurance - Specialty Industries - Property) (Details) - Property - Specialty Industries $ in Millions | Dec. 31, 2016 USD ($) reported_claim | Dec. 31, 2015 USD ($) | 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 | $ 1,279.9 |  |  |  |  |  |  |  |  |
2007 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | 94.9 | $ 95.1 | $ 95.1 | $ 95.1 | $ 95.3 | $ 95.1 | $ 94.9 | $ 101.3 | $ 104.8 | $ 110.0
Total IBNR plus expected development on reported claims | $ 0.0 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 4,828 |  |  |  |  |  |  |  |  |
2008 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 129.7 | 129.8 | 130.0 | 132.7 | 132.0 | 129.3 | 129.6 | 137.6 | $ 147.3 |
Total IBNR plus expected development on reported claims | $ 0.1 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 6,603 |  |  |  |  |  |  |  |  |
2009 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 118.6 | 118.7 | 118.6 | 118.8 | 117.9 | 118.2 | 118.6 | $ 124.5 |  |
Total IBNR plus expected development on reported claims | $ 0.1 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 6,718 |  |  |  |  |  |  |  |  |
2010 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 131.2 | 131.4 | 129.7 | 130.0 | 127.2 | 131.5 | $ 131.4 |  |  |
Total IBNR plus expected development on reported claims | $ 0.0 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 8,617 |  |  |  |  |  |  |  |  |
2011 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 142.5 | 143.2 | 143.5 | 141.1 | 141.5 | $ 138.5 |  |  |  |
Total IBNR plus expected development on reported claims | $ 0.3 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 9,704 |  |  |  |  |  |  |  |  |
2012 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 130.1 | 130.4 | 126.7 | 123.8 | $ 126.9 |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 0.8 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 9,697 |  |  |  |  |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 143.6 | 142.9 | 127.0 | $ 125.1 |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 1.9 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 8,085 |  |  |  |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 130.1 | 123.5 | $ 123.9 |  |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 3.9 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 8,352 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 138.9 | $ 142.3 |  |  |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 10.2 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 8,607 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 120.3 |  |  |  |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 32.8 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 7,429 |  |  |  |  |  |  |  |  |

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