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Michigan and Pennsylvania Operations (Details) $ in Millions |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Apr. 29, 2016 employee | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($)
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |  |
Net premiums earned |  | $ 790.9 | $ 745.5 | $ 2,337.3 | $ 2,197.8 |  |
Michigan And Pennsylvania [Member] |  |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  | $ 15.4 | $ 18.6
Combined ratio |  |  |  |  |  | 137.00% | 130.00%
Michigan And Pennsylvania [Member] | Employee Severance [Member] |  |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |  |
Number of employees impacted by workforce reduction | employee | 100 |  |  |  |  |  |
Expected employee termination costs |  | $ 2.0 |  | $ 2.0 |  |  |

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