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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, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | $ 899,304 | $ 915,012 | $ 2,633,775 | $ 2,674,034
Direct Premiums Written | 957,000 | 997,000 | 2,743,700 | 2,865,200
Private Passenger Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 653,000 | 698,200 | 1,926,500 | 2,055,100
Direct Premiums Written | 667,400 | 743,900 | 1,932,800 | 2,146,800
Homeowners Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 151,200 | 133,500 | 437,300 | 376,700
Direct Premiums Written | 183,700 | 162,900 | 511,900 | 452,800
Commercial Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 57,800 | 53,500 | 164,100 | 154,500
Direct Premiums Written | 62,000 | 55,200 | 179,500 | 165,400
Other [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 37,300 | 29,800 | 105,900 | 87,700
Direct Premiums Written | 43,900 | 35,000 | 119,500 | 100,200
Property and Casualty Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 892,000 | 907,900 | 2,612,200 | 2,652,800
Direct Premiums Written | 949,000 | 987,800 | 2,721,800 | 2,841,200
Property and Casualty Lines [Member] | Private Passenger Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 653,000 | 698,200 | 1,926,500 | 2,055,100
Direct Premiums Written | 667,400 | 743,900 | 1,932,800 | 2,146,800
Property and Casualty Lines [Member] | Homeowners Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 151,200 | 133,500 | 437,300 | 376,700
Direct Premiums Written | 183,700 | 162,900 | 511,900 | 452,800
Property and Casualty Lines [Member] | Commercial Automobile Line [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 57,800 | 53,500 | 164,100 | 154,500
Direct Premiums Written | 62,000 | 55,200 | 179,500 | 165,400
Property and Casualty Lines [Member] | Other [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 30,000 | 22,700 | 84,300 | 66,500
Direct Premiums Written | 35,900 | 25,800 | 97,600 | 76,200
Other Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 7,300 | 7,100 | 21,600 | 21,200
Direct Premiums Written | 8,000 | 9,200 | 21,900 | 24,000
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 | 7,300 | 7,100 | 21,600 | 21,200
Direct Premiums Written | $ 8,000 | $ 9,200 | $ 21,900 | $ 24,000

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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=116637232&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=116637391&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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