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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, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | $ 806,796 | $ 801,205 | $ 797,666 | $ 789,770 | $ 794,517 | $ 790,850 | $ 779,321 | $ 767,085 | $ 3,195,437 | $ 3,131,773 | $ 2,957,897
Direct premiums written |  |  |  |  |  |  |  |  | 3,248,900 | 3,172,000 | 3,012,900
Private passenger automobile [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 2,473,800 | 2,435,700 | 2,308,600
Direct premiums written |  |  |  |  |  |  |  |  | 2,480,000 | 2,452,700 | 2,345,800
Homeowners [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 431,600 | 414,000 | 379,700
Direct premiums written |  |  |  |  |  |  |  |  | 469,900 | 436,900 | 402,200
Commercial automobile [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 171,900 | 161,300 | 144,400
Direct premiums written |  |  |  |  |  |  |  |  | 178,200 | 166,100 | 153,500
Other insurance product line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 118,100 | 120,800 | 125,200
Direct premiums written |  |  |  |  |  |  |  |  | 120,800 | 116,300 | 111,400
Property and Casualty Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 3,160,900 | 3,089,900 | 2,906,600
Direct premiums written |  |  |  |  |  |  |  |  | 3,221,000 | 3,144,700 | 2,983,100
Property and Casualty Lines [Member] | Private passenger automobile [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 2,473,800 | 2,435,700 | 2,308,600
Direct premiums written |  |  |  |  |  |  |  |  | 2,480,000 | 2,452,700 | 2,345,800
Property and Casualty Lines [Member] | Homeowners [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 431,600 | 414,000 | 379,700
Direct premiums written |  |  |  |  |  |  |  |  | 469,900 | 436,900 | 402,200
Property and Casualty Lines [Member] | Commercial automobile [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 171,900 | 161,300 | 144,400
Direct premiums written |  |  |  |  |  |  |  |  | 178,200 | 166,100 | 153,500
Property and Casualty Lines [Member] | Other insurance product line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 83,600 | 78,900 | 73,900
Direct premiums written |  |  |  |  |  |  |  |  | 92,900 | 89,000 | 81,600
Other Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned |  |  |  |  |  |  |  |  | 34,500 | 41,900 | 51,300
Direct premiums written |  |  |  |  |  |  |  |  | 27,900 | 27,300 | 29,800
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 |  |  |  |  |  |  |  |  | 34,500 | 41,900 | 51,300
Direct premiums written |  |  |  |  |  |  |  |  | $ 27,900 | $ 27,300 | $ 29,800

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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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b) -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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(c)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915 + Details
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