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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, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | $ 3,741,948 | $ 3,555,635 | $ 3,599,418
Direct premiums written | 3,907,500 | 3,654,200 | 3,774,600
Private passenger automobile |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 2,613,500 | 2,591,300 | 2,756,500
Direct premiums written | 2,640,700 | 2,569,000 | 2,820,500
Homeowners |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 698,100 | 601,300 | 514,800
Direct premiums written | 801,500 | 678,900 | 598,500
Commercial automobile |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 258,600 | 225,600 | 208,700
Direct premiums written | 259,900 | 240,800 | 217,300
Other |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 171,700 | 137,400 | 119,400
Direct premiums written | 205,400 | 165,500 | 138,300
Property & Casualty |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 3,712,910 | 3,526,753 | 3,571,042
Direct premiums written | 3,877,476 | 3,624,965 | 3,742,627
Property & Casualty | Private passenger automobile |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 2,613,500 | 2,591,300 | 2,756,500
Direct premiums written | 2,640,700 | 2,569,000 | 2,820,500
Property & Casualty | Homeowners |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 698,100 | 601,300 | 514,800
Direct premiums written | 801,500 | 678,900 | 598,500
Property & Casualty | Commercial automobile |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 258,600 | 225,600 | 208,700
Direct premiums written | 259,900 | 240,800 | 217,300
Property & Casualty | Other |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 142,700 | 108,600 | 91,000
Direct premiums written | 175,400 | 136,300 | 106,300
Other |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 29,000 | 28,800 | 28,400
Direct premiums written | 30,000 | 29,200 | 32,000
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 | 28,800 | 28,400
Direct premiums written | $ 30,000 | $ 29,200 | $ 32,000

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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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