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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, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | $ 915,012 | $ 858,135 | $ 2,674,034 | $ 2,500,178
Direct Premiums Written | 997,000 | 917,500 | 2,865,200 | 2,673,100
Private Passenger Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 698,200 | 659,600 | 2,055,100 | 1,932,100
Direct Premiums Written | 743,900 | 695,100 | 2,146,800 | 2,035,100
Homeowners Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 133,500 | 120,000 | 376,700 | 340,500
Direct Premiums Written | 162,900 | 140,700 | 452,800 | 394,000
Commercial Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 53,500 | 48,700 | 154,500 | 140,300
Direct Premiums Written | 55,200 | 50,300 | 165,400 | 149,900
Other [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 29,800 | 29,800 | 87,700 | 87,300
Direct Premiums Written | 35,000 | 31,400 | 100,200 | 94,100
Property and Casualty Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 907,900 | 850,500 | 2,652,800 | 2,476,900
Direct Premiums Written | 987,800 | 910,600 | 2,841,200 | 2,652,600
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 698,200 | 659,600 | 2,055,100 | 1,932,100
Direct Premiums Written | 743,900 | 695,100 | 2,146,800 | 2,035,100
Property and Casualty Lines [Member] | Homeowners Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 133,500 | 120,000 | 376,700 | 340,500
Direct Premiums Written | 162,900 | 140,700 | 452,800 | 394,000
Property and Casualty Lines [Member] | Commercial Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 53,500 | 48,700 | 154,500 | 140,300
Direct Premiums Written | 55,200 | 50,300 | 165,400 | 149,900
Property and Casualty Lines [Member] | Other [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 22,700 | 22,200 | 66,500 | 64,000
Direct Premiums Written | 25,800 | 24,500 | 76,200 | 73,600
Other Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 7,100 | 7,600 | 21,200 | 23,300
Direct Premiums Written | 9,200 | 6,900 | 24,000 | 20,500
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 | 7,100 | 7,600 | 21,200 | 23,300
Direct Premiums Written | $ 9,200 | $ 6,900 | $ 24,000 | $ 20,500

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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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