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Reserves for Unpaid Losses and Loss Adjustment Expenses (Cumulative Paid Loss and Allocated LAE, Net of Reinsurance - Specialty Industries - Property) (Details) - Property - Specialty Industries - USD ($) $ in Millions | 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, 2008 | Dec. 31, 2007
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Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | $ 1,171.4 |  |  |  |  |  |  |  |  |
All outstanding liabilities, net of reinsurance | 0.0 |  |  |  |  |  |  |  |  |
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance | 108.5 |  |  |  |  |  |  |  |  |
2007 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 94.6 | $ 94.5 | $ 94.5 | $ 94.2 | $ 93.4 | $ 91.7 | $ 87.6 | $ 82.7 | $ 70.9 | $ 42.2
2008 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 129.3 | 129.3 | 129.2 | 129.1 | 129.5 | 123.4 | 116.8 | 101.4 | $ 62.5 |
2009 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 118.4 | 118.0 | 117.2 | 115.5 | 111.0 | 105.4 | 85.9 | $ 50.4 |  |
2010 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 130.8 | 129.3 | 125.9 | 122.8 | 118.2 | 106.2 | $ 69.0 |  |  |
2011 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 141.4 | 139.5 | 138.0 | 130.7 | 117.4 | $ 72.5 |  |  |  |
2012 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 131.9 | 128.5 | 116.4 | 103.1 | $ 64.2 |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 137.7 | 126.1 | 101.5 | $ 59.8 |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 111.6 | 94.6 | $ 56.5 |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 114.4 | $ 68.6 |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | $ 61.3 |  |  |  |  |  |  |  |  |

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