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Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2022 | Mar. 31, 2021
Segment Reporting Information [Line Items] |  |
Net premiums earned | $ 962,550 | $ 915,922
Direct premiums written | 1,018,200 | 953,400
Private passenger automobile |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 659,300 | 653,700
Direct premiums written | 681,300 | 666,500
Homeowners |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 192,000 | 160,800
Direct premiums written | 210,200 | 170,200
Commercial automobile |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 64,500 | 62,600
Direct premiums written | 71,600 | 68,900
Other |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 46,800 | 38,800
Direct premiums written | 55,100 | 47,800
Property & Casualty |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 955,418 | 908,815
Direct premiums written | 1,011,385 | 946,606
Property & Casualty | Private passenger automobile |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 659,300 | 653,700
Direct premiums written | 681,300 | 666,500
Property & Casualty | Homeowners |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 192,000 | 160,800
Direct premiums written | 210,200 | 170,200
Property & Casualty | Commercial automobile |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 64,500 | 62,600
Direct premiums written | 71,600 | 68,900
Property & Casualty | Other |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 39,600 | 31,700
Direct premiums written | 48,300 | 41,000
Other |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 7,200 | 7,100
Direct premiums written | 6,800 | 6,800
Other | Private passenger automobile |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 0 | 0
Direct premiums written | 0 | 0
Other | Homeowners |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 0 | 0
Direct premiums written | 0 | 0
Other | Commercial automobile |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 0 | 0
Direct premiums written | 0 | 0
Other | Other |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 7,200 | 7,100
Direct premiums written | $ 6,800 | $ 6,800

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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=120400993&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=120401096&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=120399700&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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