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Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | $ 858,135 | $ 801,205 | $ 2,500,178 | $ 2,388,641
Direct Premiums Written | 917,500 | 835,100 | 2,673,100 | 2,450,000
Private Passenger Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 659,600 | 619,800 | 1,932,100 | 1,850,300
Direct Premiums Written | 695,100 | 633,700 | 2,035,100 | 1,872,800
Homeowners Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 120,000 | 108,300 | 340,500 | 322,300
Direct Premiums Written | 140,700 | 125,800 | 394,000 | 352,300
Commercial Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 48,700 | 43,600 | 140,300 | 127,300
Direct Premiums Written | 50,300 | 45,600 | 149,900 | 133,700
Other [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 29,800 | 29,500 | 87,300 | 88,700
Direct Premiums Written | 31,400 | 30,000 | 94,100 | 91,200
Property and Casualty Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 850,500 | 792,700 | 2,476,900 | 2,362,400
Direct Premiums Written | 910,600 | 828,000 | 2,652,600 | 2,428,700
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 659,600 | 619,800 | 1,932,100 | 1,850,300
Direct Premiums Written | 695,100 | 633,700 | 2,035,100 | 1,872,800
Property and Casualty Lines [Member] | Homeowners Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 120,000 | 108,300 | 340,500 | 322,300
Direct Premiums Written | 140,700 | 125,800 | 394,000 | 352,300
Property and Casualty Lines [Member] | Commercial Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 48,700 | 43,600 | 140,300 | 127,300
Direct Premiums Written | 50,300 | 45,600 | 149,900 | 133,700
Property and Casualty Lines [Member] | Other [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 22,200 | 21,000 | 64,000 | 62,500
Direct Premiums Written | 24,500 | 22,900 | 73,600 | 69,900
Other Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 7,600 | 8,500 | 23,300 | 26,200
Direct Premiums Written | 6,900 | 7,100 | 20,500 | 21,300
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 | 7,600 | 8,500 | 23,300 | 26,200
Direct Premiums Written | $ 6,900 | $ 7,100 | $ 20,500 | $ 21,300

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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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(1)) -URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17(Column E)) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(c)) -URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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