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Segment Information - Summary of Premiums Written and Earned by Segment (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | $ 925,384 | $ 915,012 | $ 888,776 | $ 870,245 | $ 868,233 | $ 858,135 | $ 833,959 | $ 808,084 | $ 3,599,418 | $ 3,368,411 | $ 3,195,437
Direct premiums written |  |  |  |  |  |  |  |  | 3,774,600 | 3,550,900 | 3,248,900
Private passenger automobile [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 2,756,500 | 2,602,100 | 2,473,800
Direct premiums written |  |  |  |  |  |  |  |  | 2,820,500 | 2,703,600 | 2,480,000
Homeowners [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 514,800 | 459,400 | 431,600
Direct premiums written |  |  |  |  |  |  |  |  | 598,500 | 524,900 | 469,900
Commercial automobile [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 208,700 | 190,100 | 171,900
Direct premiums written |  |  |  |  |  |  |  |  | 217,300 | 198,500 | 178,200
Other insurance product line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 119,400 | 116,800 | 118,100
Direct premiums written |  |  |  |  |  |  |  |  | 138,300 | 123,900 | 120,800
Property and Casualty Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 3,571,000 | 3,337,900 | 3,160,900
Direct premiums written |  |  |  |  |  |  |  |  | 3,742,600 | 3,524,200 | 3,221,000
Property and Casualty Lines [Member] | Private passenger automobile [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 2,756,500 | 2,602,100 | 2,473,800
Direct premiums written |  |  |  |  |  |  |  |  | 2,820,500 | 2,703,600 | 2,480,000
Property and Casualty Lines [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 514,800 | 459,400 | 431,600
Direct premiums written |  |  |  |  |  |  |  |  | 598,500 | 524,900 | 469,900
Property and Casualty Lines [Member] | Commercial automobile [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 208,700 | 190,100 | 171,900
Direct premiums written |  |  |  |  |  |  |  |  | 217,300 | 198,500 | 178,200
Property and Casualty Lines [Member] | Other insurance product line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 91,000 | 86,300 | 83,600
Direct premiums written |  |  |  |  |  |  |  |  | 106,300 | 97,200 | 92,900
Other Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 28,400 | 30,500 | 34,500
Direct premiums written |  |  |  |  |  |  |  |  | 32,000 | 26,700 | 27,900
Other Segments [Member] | Private passenger automobile [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 0 | 0 | 0
Direct premiums written |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other Segments [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 0 | 0 | 0
Direct premiums written |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other Segments [Member] | Commercial automobile [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 0 | 0 | 0
Direct premiums written |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other Segments [Member] | Other insurance product line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 28,400 | 30,500 | 34,500
Direct premiums written |  |  |  |  |  |  |  |  | $ 32,000 | $ 26,700 | $ 27,900

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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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