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Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | $ 5,075,456 | $ 4,274,378 | $ 3,952,482
Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 5,075,500 | 4,274,400 | 3,952,500
Direct premiums written | 5,500,800 | 4,558,500 | 4,048,500
Private passenger automobile | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 3,278,500 | 2,796,100 | 2,655,700
Direct premiums written | 3,395,800 | 2,841,800 | 2,598,900
Homeowners | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,168,700 | 957,700 | 830,600
Direct premiums written | 1,367,300 | 1,119,000 | 940,300
Commercial automobile | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 378,200 | 303,900 | 267,900
Direct premiums written | 387,000 | 346,200 | 276,800
Other | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 250,100 | 216,700 | 198,300
Direct premiums written | 350,700 | 251,500 | 232,500
Property & Casualty |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 5,046,099 | 4,245,369 | 3,923,642
Direct premiums written | 5,474,292 | 4,531,942 | 4,018,057
Property & Casualty | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 5,046,100 | 4,245,400 | 3,923,600
Direct premiums written | 5,474,300 | 4,531,900 | 4,018,100
Property & Casualty | Private passenger automobile | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 3,278,500 | 2,796,100 | 2,655,700
Direct premiums written | 3,395,800 | 2,841,800 | 2,598,900
Property & Casualty | Homeowners | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,168,700 | 957,700 | 830,600
Direct premiums written | 1,367,300 | 1,119,000 | 940,300
Property & Casualty | Commercial automobile | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 378,200 | 303,900 | 267,900
Direct premiums written | 387,000 | 346,200 | 276,800
Property & Casualty | Other | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 220,700 | 187,700 | 169,400
Direct premiums written | 324,200 | 224,900 | 202,100
Other | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 29,400 | 29,000 | 28,900
Direct premiums written | 26,500 | 26,600 | 30,400
Other | Private passenger automobile | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 0 | 0 | 0
Direct premiums written | 0 | 0 | 0
Other | Homeowners | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 0 | 0 | 0
Direct premiums written | 0 | 0 | 0
Other | Commercial automobile | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 0 | 0 | 0
Direct premiums written | 0 | 0 | 0
Other | Other | Operating Segments |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 29,400 | 29,000 | 28,900
Direct premiums written | $ 26,500 | $ 26,600 | $ 30,400

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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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