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Reserves for Unpaid Losses and Loss Adjustment Expenses (Incurred Loss and Allocated LAE, Net of Reinsurance - Specialty Products - Casualty) (Details) - Casualty - 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 | $ 1,976.1 |  |  |  |  |  |  |  |  |
2007 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 1,236 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 56.5 | $ 57.1 | $ 58.3 | $ 61.8 | $ 70.1 | $ 70.5 | $ 90.7 | $ 97.3 | $ 120.2 | $ 138.3
Total IBNR plus expected development on reported claims | $ 2.5 |  |  |  |  |  |  |  |  |
2008 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 2,199 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 96.8 | 97.3 | 95.3 | 95.5 | 98.3 | 100.8 | 118.2 | 128.6 | $ 145.6 |
Total IBNR plus expected development on reported claims | $ 3.3 |  |  |  |  |  |  |  |  |
2009 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 4,002 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 195.5 | 195.0 | 194.9 | 192.0 | 193.2 | 194.9 | 186.2 | $ 169.5 |  |
Total IBNR plus expected development on reported claims | $ 4.9 |  |  |  |  |  |  |  |  |
2010 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 6,029 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 240.0 | 234.9 | 232.3 | 225.0 | 226.2 | 225.4 | $ 211.5 |  |  |
Total IBNR plus expected development on reported claims | $ 7.8 |  |  |  |  |  |  |  |  |
2011 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 6,661 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 214.9 | 214.8 | 214.2 | 205.4 | 189.8 | $ 194.9 |  |  |  |
Total IBNR plus expected development on reported claims | $ 9.4 |  |  |  |  |  |  |  |  |
2012 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 6,794 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 249.4 | 248.8 | 246.1 | 220.1 | $ 213.5 |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 14.4 |  |  |  |  |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 5,866 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 247.9 | 229.6 | 233.1 | $ 205.2 |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 20.3 |  |  |  |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 6,504 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 258.9 | 235.9 | $ 240.5 |  |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 44.4 |  |  |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 9,710 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 213.4 | $ 206.9 |  |  |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 78.5 |  |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative number of reported claims | reported_claim | 9,093 |  |  |  |  |  |  |  |  |
Incurred Loss and Allocated LAE, Net of Reinsurance | $ 202.8 |  |  |  |  |  |  |  |  |
Total IBNR plus expected development on reported claims | $ 142.8 |  |  |  |  |  |  |  |  |

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