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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, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | $ 1,090,311 | $ 996,939 | $ 3,129,483 | $ 2,947,000
Direct premiums written | 1,237,200 | 1,057,100 | 3,395,200 | 3,109,500
Private passenger automobile |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 713,200 | 665,600 | 2,048,500 | 1,990,900
Direct premiums written | 772,800 | 680,600 | 2,105,600 | 2,015,600
Homeowners |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 244,000 | 213,000 | 700,400 | 610,900
Direct premiums written | 307,700 | 251,800 | 843,800 | 713,900
Commercial automobile |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 78,400 | 68,000 | 219,600 | 198,600
Direct premiums written | 93,700 | 66,500 | 256,400 | 207,900
Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 54,700 | 50,300 | 161,000 | 146,600
Direct premiums written | 63,000 | 58,200 | 189,400 | 172,100
Property & Casualty |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 1,082,979 | 989,688 | 3,107,812 | 2,925,410
Direct premiums written | 1,230,024 | 1,049,088 | 3,374,477 | 3,086,222
Property & Casualty | Private passenger automobile |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 713,200 | 665,600 | 2,048,500 | 1,990,900
Direct premiums written | 772,800 | 680,600 | 2,105,600 | 2,015,600
Property & Casualty | Homeowners |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 244,000 | 213,000 | 700,400 | 610,900
Direct premiums written | 307,700 | 251,800 | 843,800 | 713,900
Property & Casualty | Commercial automobile |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 78,400 | 68,000 | 219,600 | 198,600
Direct premiums written | 93,700 | 66,500 | 256,400 | 207,900
Property & Casualty | Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 47,400 | 43,100 | 139,300 | 125,000
Direct premiums written | 55,800 | 50,200 | 168,700 | 148,800
Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 7,300 | 7,200 | 21,700 | 21,600
Direct premiums written | 7,200 | 8,000 | 20,700 | 23,300
Other | Private passenger automobile |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 0 | 0 | 0 | 0
Direct premiums written | 0 | 0 | 0 | 0
Other | Homeowners |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 0 | 0 | 0 | 0
Direct premiums written | 0 | 0 | 0 | 0
Other | Commercial automobile |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 0 | 0 | 0 | 0
Direct premiums written | 0 | 0 | 0 | 0
Other | Other |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Net premiums earned | 7,300 | 7,200 | 21,700 | 21,600
Direct premiums written | $ 7,200 | $ 8,000 | $ 20,700 | $ 23,300

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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 -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479655/944-605-50-1 + 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 -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17(Column E)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480149/944-235-S99-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479655/944-605-50-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(c)) -SubTopic 220 -Topic 942 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483589/942-220-S99-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(1)) -SubTopic 220 -Topic 944 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483586/944-220-S99-1 + Details
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