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Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | $ 790.9 | $ 745.5 | $ 2,337.3 | $ 2,197.8
Direct Premiums Written | 817.0 | 785.4 | 2,401.1 | 2,264.1
Property and Casualty, Personal Insurance Product Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 614.0 | 581.3 | 1,820.1 | 1,716.0
Direct Premiums Written | 630.1 | 609.0 | 1,857.3 | 1,763.1
Property Insurance Product Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 105.5 | 95.8 | 307.0 | 281.6
Direct Premiums Written | 116.0 | 106.6 | 331.4 | 300.8
Property and Casualty, Commercial Insurance Product Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 41.2 | 37.0 | 119.8 | 106.3
Direct Premiums Written | 41.9 | 40.1 | 125.2 | 114.8
Other Insurance Product Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 30.2 | 31.4 | 90.4 | 93.9
Direct Premiums Written | 29.0 | 29.7 | 87.2 | 85.4
Property and Casualty Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 780.6 | 732.9 | 2,305.0 | 2,158.6
Direct Premiums Written | 810.4 | 778.0 | 2,380.2 | 2,241.3
Property and Casualty Lines [Member] | Property and Casualty, Personal Insurance Product Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 614.0 | 581.3 | 1,820.1 | 1,716.0
Direct Premiums Written | 630.1 | 609.0 | 1,857.3 | 1,763.1
Property and Casualty Lines [Member] | Property Insurance Product Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 105.5 | 95.8 | 307.0 | 281.6
Direct Premiums Written | 116.0 | 106.6 | 331.4 | 300.8
Property and Casualty Lines [Member] | Property and Casualty, Commercial Insurance Product Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 41.2 | 37.0 | 119.8 | 106.3
Direct Premiums Written | 41.9 | 40.1 | 125.2 | 114.8
Property and Casualty Lines [Member] | Other Insurance Product Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 19.9 | 18.8 | 58.1 | 54.7
Direct Premiums Written | 22.4 | 22.3 | 66.3 | 62.6
Other Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 10.3 | 12.6 | 32.3 | 39.2
Direct Premiums Written | 6.6 | 7.4 | 20.9 | 22.8
Other Segments [Member] | Property and Casualty, Personal Insurance Product Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 0.0 | 0.0 | 0.0 | 0.0
Direct Premiums Written | 0.0 | 0.0 | 0.0 | 0.0
Other Segments [Member] | Property Insurance Product Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 0.0 | 0.0 | 0.0 | 0.0
Direct Premiums Written | 0.0 | 0.0 | 0.0 | 0.0
Other Segments [Member] | Property and Casualty, Commercial Insurance Product Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 0.0 | 0.0 | 0.0 | 0.0
Direct Premiums Written | 0.0 | 0.0 | 0.0 | 0.0
Other Segments [Member] | Other Insurance Product Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 10.3 | 12.6 | 32.3 | 39.2
Direct Premiums Written | $ 6.6 | $ 7.4 | $ 20.9 | $ 22.8

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