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Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2017 | Mar. 31, 2016
Segment Reporting Information [Line Items] |  |
Net premiums earned | $ 789.8 | $ 767.1
Direct Premiums Written | 814.2 | 800.1
Private Passenger Automobile Line [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 613.3 | 599.0
Direct Premiums Written | 639.0 | 631.8
Homeowners Line [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 105.7 | 99.5
Direct Premiums Written | 103.8 | 99.1
Commercial Automobile Line [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 41.3 | 38.7
Direct Premiums Written | 43.0 | 40.9
Other [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 29.5 | 29.9
Direct Premiums Written | 28.4 | 28.3
Property and Casualty Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 780.8 | 755.8
Direct Premiums Written | 808.5 | 792.7
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 613.3 | 599.0
Direct Premiums Written | 639.0 | 631.8
Property and Casualty Lines [Member] | Homeowners Line [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 105.7 | 99.5
Direct Premiums Written | 103.8 | 99.1
Property and Casualty Lines [Member] | Commercial Automobile Line [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 41.3 | 38.7
Direct Premiums Written | 43.0 | 40.9
Property and Casualty Lines [Member] | Other [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 20.5 | 18.6
Direct Premiums Written | 22.7 | 20.9
Other Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 9.0 | 11.3
Direct Premiums Written | 5.7 | 7.4
Other Segments [Member] | Private Passenger Automobile Line [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 0.0 | 0.0
Direct Premiums Written | 0.0 | 0.0
Other Segments [Member] | Homeowners Line [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 0.0 | 0.0
Direct Premiums Written | 0.0 | 0.0
Other Segments [Member] | Commercial Automobile Line [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 0.0 | 0.0
Direct Premiums Written | 0.0 | 0.0
Other Segments [Member] | Other [Member] |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 9.0 | 11.3
Direct Premiums Written | $ 5.7 | $ 7.4

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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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- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(c)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915 + Details
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