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Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Segment Reporting Information [Line Items] |  |  |  |
Document Period End Date |  |  | Jun. 30, 2017 |
Net premiums earned | $ 797.7 | $ 779.3 | $ 1,587.4 | $ 1,546.4
Direct Premiums Written | 800.6 | 784.0 | 1,614.9 | 1,584.1
Private Passenger Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 617.3 | 607.1 | 1,230.5 | 1,206.1
Direct Premiums Written | 600.1 | 595.4 | 1,239.1 | 1,227.2
Homeowners Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 108.3 | 102.0 | 214.0 | 201.5
Direct Premiums Written | 122.7 | 116.3 | 226.5 | 215.4
Commercial Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 42.4 | 39.9 | 83.7 | 78.6
Direct Premiums Written | 45.1 | 42.4 | 88.1 | 83.3
Other [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 29.7 | 30.3 | 59.2 | 60.2
Direct Premiums Written | 32.7 | 29.9 | 61.2 | 58.2
Property and Casualty Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 789.0 | 768.6 | 1,569.7 | 1,524.4
Direct Premiums Written | 792.1 | 777.1 | 1,600.7 | 1,569.8
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 617.3 | 607.1 | 1,230.5 | 1,206.1
Direct Premiums Written | 600.1 | 595.4 | 1,239.1 | 1,227.2
Property and Casualty Lines [Member] | Homeowners Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 108.3 | 102.0 | 214.0 | 201.5
Direct Premiums Written | 122.7 | 116.3 | 226.5 | 215.4
Property and Casualty Lines [Member] | Commercial Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 42.4 | 39.9 | 83.7 | 78.6
Direct Premiums Written | 45.1 | 42.4 | 88.1 | 83.3
Property and Casualty Lines [Member] | Other [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 21.0 | 19.6 | 41.5 | 38.2
Direct Premiums Written | 24.2 | 23.0 | 47.0 | 43.9
Other Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 8.7 | 10.7 | 17.7 | 22.0
Direct Premiums Written | 8.5 | 6.9 | 14.2 | 14.3
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 | 8.7 | 10.7 | 17.7 | 22.0
Direct Premiums Written | $ 8.5 | $ 6.9 | $ 14.2 | $ 14.3

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- Definition The end date of the period reflected on the cover page if a periodic report. For all other reports and registration statements containing historical data, it is the date up through which that historical data is presented. If there is no historical data in the report, use the filing date. The format of the date is CCYY-MM-DD. + References No definition available. + Details
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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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