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Reserves for Unpaid Losses and Loss Adjustment Expenses (Cumulative Paid Loss and Allocated LAE, Net of Reinsurance - Specialty Products - Casualty) (Details) - Casualty - Specialty Products - 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 | $ 1,411.7 |  |  |  |  |  |  |  |  |
All outstanding liabilities, net of reinsurance | 4.3 |  |  |  |  |  |  |  |  |
Total unpaid and undiscounted loss and allocated LAE reserves, net of reinsurance | 568.7 |  |  |  |  |  |  |  |  |
2007 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 54.0 | $ 54.0 | $ 53.6 | $ 51.9 | $ 50.7 | $ 44.4 | $ 41.7 | $ 32.4 | $ 24.8 | $ 6.8
2008 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 91.5 | 88.1 | 85.3 | 83.1 | 77.6 | 65.4 | 50.8 | 32.3 | $ 8.8 |
2009 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 187.8 | 178.3 | 169.9 | 161.4 | 147.8 | 124.0 | 81.8 | $ 26.5 |  |
2010 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 220.3 | 213.6 | 196.3 | 180.5 | 157.8 | 107.0 | $ 32.0 |  |  |
2011 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 192.5 | 186.2 | 163.7 | 131.3 | 88.9 | $ 25.8 |  |  |  |
2012 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 219.8 | 193.6 | 157.7 | 86.7 | $ 25.8 |  |  |  |  |
2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 194.2 | 143.1 | 86.3 | $ 26.4 |  |  |  |  |  |
2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 156.7 | 95.2 | $ 34.3 |  |  |  |  |  |  |
2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | 78.4 | $ 19.9 |  |  |  |  |  |  |  |
2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and Allocated LAE, Net of Reinsurance | $ 16.5 |  |  |  |  |  |  |  |  |

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