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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, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | $ 801.2 | $ 790.9 | $ 2,388.6 | $ 2,337.3
Direct Premiums Written | 835.1 | 817.0 | 2,450.0 | 2,401.1
Private Passenger Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 619.8 | 614.0 | 1,850.3 | 1,820.1
Direct Premiums Written | 633.7 | 630.1 | 1,872.8 | 1,857.3
Homeowners Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 108.3 | 105.5 | 322.3 | 307.0
Direct Premiums Written | 125.8 | 116.0 | 352.3 | 331.4
Commercial Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 43.6 | 41.2 | 127.3 | 119.8
Direct Premiums Written | 45.6 | 41.9 | 133.7 | 125.2
Other [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 29.5 | 30.2 | 88.7 | 90.4
Direct Premiums Written | 30.0 | 29.0 | 91.2 | 87.2
Property and Casualty Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 792.7 | 780.6 | 2,362.4 | 2,305.0
Direct Premiums Written | 828.0 | 810.4 | 2,428.7 | 2,380.2
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 619.8 | 614.0 | 1,850.3 | 1,820.1
Direct Premiums Written | 633.7 | 630.1 | 1,872.8 | 1,857.3
Property and Casualty Lines [Member] | Homeowners Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 108.3 | 105.5 | 322.3 | 307.0
Direct Premiums Written | 125.8 | 116.0 | 352.3 | 331.4
Property and Casualty Lines [Member] | Commercial Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 43.6 | 41.2 | 127.3 | 119.8
Direct Premiums Written | 45.6 | 41.9 | 133.7 | 125.2
Property and Casualty Lines [Member] | Other [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 21.0 | 19.9 | 62.5 | 58.1
Direct Premiums Written | 22.9 | 22.4 | 69.9 | 66.3
Other Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 8.5 | 10.3 | 26.2 | 32.3
Direct Premiums Written | 7.1 | 6.6 | 21.3 | 20.9
Other Segments [Member] | Private Passenger Automobile 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] | Homeowners 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] | Commercial Automobile 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 [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 8.5 | 10.3 | 26.2 | 32.3
Direct Premiums Written | $ 7.1 | $ 6.6 | $ 21.3 | $ 20.9

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- Definition Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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