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Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Net premiums earned	$ 4,274,378	$ 3,952,482	$ 3,741,948
Direct premiums written	4,558,500	4,048,500	3,907,500
Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	2,796,100	2,655,700	2,613,500
Direct premiums written	2,841,800	2,598,900	2,640,700
Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	957,700	830,600	698,100
Direct premiums written	1,119,000	940,300	801,500
Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	303,900	267,900	258,600
Direct premiums written	346,200	276,800	259,900
Other
Segment Reporting Information [Line Items]
Net premiums earned	216,700	198,300	171,700
Direct premiums written	251,500	232,500	205,400
Property & Casualty
Segment Reporting Information [Line Items]
Net premiums earned	4,245,369	3,923,642	3,712,910
Direct premiums written	4,531,942	4,018,057	3,877,476
Property & Casualty | Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	2,796,100	2,655,700	2,613,500
Direct premiums written	2,841,800	2,598,900	2,640,700
Property & Casualty | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	957,700	830,600	698,100
Direct premiums written	1,119,000	940,300	801,500
Property & Casualty | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	303,900	267,900	258,600
Direct premiums written	346,200	276,800	259,900
Property & Casualty | Other
Segment Reporting Information [Line Items]
Net premiums earned	187,700	169,400	142,700
Direct premiums written	224,900	202,100	175,400
Other
Segment Reporting Information [Line Items]
Net premiums earned	29,000	28,900	29,000
Direct premiums written	26,600	30,400	30,000
Other | Private passenger automobile
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0
Direct premiums written	0	0	0
Other | Homeowners
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0
Direct premiums written	0	0	0
Other | Commercial automobile
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0
Direct premiums written	0	0	0
Other | Other
Segment Reporting Information [Line Items]
Net premiums earned	29,000	28,900	29,000
Direct premiums written	$ 26,600	$ 30,400	$ 30,000

X

- 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

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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

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