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Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred Loss and Allocated LAE, Net of Reinsurance - Specialty Products - Property) (Details) - Property - Specialty Products $ 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 | $ 405.2 |  |  |  |  |  |  |  |  |
2007 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | 8.6 | $ 8.6 | $ 8.6 | $ 8.7 | $ 7.7 | $ 7.6 | $ 7.6 | $ 9.1 | $ 11.1 | $ 12.0
Total IBNR plus expected development on reported claims | $ 0.0 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 250 |  |  |  |  |  |  |  |  |
2008 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 26.8 | 26.8 | 26.8 | 26.8 | 26.6 | 25.6 | 25.4 | 29.2 | $ 31.3 |
Total IBNR plus expected development on reported claims | $ 0.0 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 3,466 |  |  |  |  |  |  |  |  |
2009 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 51.6 | 52.5 | 52.6 | 52.9 | 53.3 | 52.9 | 52.1 | $ 55.2 |  |
Total IBNR plus expected development on reported claims | $ 0.0 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 8,768 |  |  |  |  |  |  |  |  |
2010 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 55.8 | 55.8 | 55.9 | 56.7 | 55.6 | 49.5 | $ 51.0 |  |  |
Total IBNR plus expected development on reported claims | $ 0.0 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 10,170 |  |  |  |  |  |  |  |  |
2011 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 60.7 | 60.8 | 60.6 | 60.2 | 59.8 | $ 58.7 |  |  |  |
Total IBNR plus expected development on reported claims | $ 0.2 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 11,121 |  |  |  |  |  |  |  |  |
2012 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 92.2 | 92.2 | 92.9 | 86.7 | $ 79.9 |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 0.2 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 12,405 |  |  |  |  |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 27.7 | 27.6 | 27.6 | $ 36.4 |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 0.3 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 5,267 |  |  |  |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 31.6 | 32.1 | $ 37.5 |  |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 0.4 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 1,227 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 25.7 | $ 28.3 |  |  |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 1.1 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 2,186 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 24.5 |  |  |  |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 5.2 |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 1,825 |  |  |  |  |  |  |  |  |

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