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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, 2025 | Mar. 31, 2024
Segment Reporting Information [Line Items] |  |
Net premiums earned | $ 1,283,069 | $ 1,166,679
Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 1,283,100 | 1,166,700
Direct premiums written | 1,445,400 | 1,301,100
Private passenger automobile | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 858,600 | 754,300
Direct premiums written | 906,500 | 825,500
Homeowners | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 256,000 | 266,900
Direct premiums written | 344,200 | 302,100
Commercial automobile | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 95,300 | 89,200
Direct premiums written | 106,500 | 104,000
Other | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 73,200 | 56,300
Direct premiums written | 88,200 | 69,500
Property & Casualty |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 1,275,758 | 1,159,369
Direct premiums written | 1,438,467 | 1,294,475
Property & Casualty | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 1,275,800 | 1,159,400
Direct premiums written | 1,438,500 | 1,294,500
Property & Casualty | Private passenger automobile | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 858,600 | 754,300
Direct premiums written | 906,500 | 825,500
Property & Casualty | Homeowners | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 256,000 | 266,900
Direct premiums written | 344,200 | 302,100
Property & Casualty | Commercial automobile | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 95,300 | 89,200
Direct premiums written | 106,500 | 104,000
Property & Casualty | Other | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 65,900 | 49,000
Direct premiums written | 81,300 | 62,900
Other | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 7,300 | 7,300
Direct premiums written | 6,900 | 6,600
Other | Private passenger automobile | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 0 | 0
Direct premiums written | 0 | 0
Other | Homeowners | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 0 | 0
Direct premiums written | 0 | 0
Other | Commercial automobile | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 0 | 0
Direct premiums written | 0 | 0
Other | Other | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net premiums earned | 7,300 | 7,300
Direct premiums written | $ 6,900 | $ 6,600

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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/1943274/2147479032/944-605-50-1 + 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/1943274/2147477965/944-235-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/1943274/2147479032/944-605-50-1 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/1943274/2147478524/942-220-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/1943274/2147477250/944-220-S99-1 + Details
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