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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, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | $ 3,012,900 | $ 2,853,200 | $ 2,737,500
Net premiums earned | $ 760,094 | $ 745,520 | $ 731,546 | $ 720,737 | $ 709,368 | $ 705,237 | $ 697,889 | $ 683,701 | 2,957,897 | 2,796,195 | 2,698,187
Property and Casualty, Personal Insurance Product Line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | 2,345,800 | 2,223,100 | 2,165,600
Net premiums earned |  |  |  |  |  |  |  |  | 2,308,600 | 2,203,000 | 2,163,400
Property Insurance Product Line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | 402,200 | 374,500 | 340,000
Net premiums earned |  |  |  |  |  |  |  |  | 379,700 | 347,900 | 328,500
Property and Casualty, Commercial Insurance Product Line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | 153,500 | 135,900 | 104,700
Net premiums earned |  |  |  |  |  |  |  |  | 144,400 | 121,800 | 88,300
Other Insurance Product Line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | 111,400 | 119,700 | 127,200
Net premiums earned |  |  |  |  |  |  |  |  | 125,200 | 123,500 | 118,000
Property and Casualty Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | 2,983,100 | 2,808,900 | 2,674,700
Net premiums earned |  |  |  |  |  |  |  |  | 2,906,600 | 2,737,300 | 2,638,400
Property and Casualty Lines [Member] | Property and Casualty, Personal Insurance Product Line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | 2,345,800 | 2,223,100 | 2,165,600
Net premiums earned |  |  |  |  |  |  |  |  | 2,308,600 | 2,203,000 | 2,163,400
Property and Casualty Lines [Member] | Property Insurance Product Line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | 402,200 | 374,500 | 340,000
Net premiums earned |  |  |  |  |  |  |  |  | 379,700 | 347,900 | 328,500
Property and Casualty Lines [Member] | Property and Casualty, Commercial Insurance Product Line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | 153,500 | 135,900 | 104,700
Net premiums earned |  |  |  |  |  |  |  |  | 144,400 | 121,800 | 88,300
Property and Casualty Lines [Member] | Other Insurance Product Line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | 81,600 | 75,400 | 64,400
Net premiums earned |  |  |  |  |  |  |  |  | 73,900 | 64,600 | 58,200
Other Segments [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | 29,800 | 44,300 | 62,800
Net premiums earned |  |  |  |  |  |  |  |  | 51,300 | 58,900 | 59,800
Other Segments [Member] | Property and Casualty, Personal Insurance Product Line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net premiums earned |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other Segments [Member] | Property Insurance Product Line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net premiums earned |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other Segments [Member] | Property and Casualty, Commercial Insurance Product Line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net premiums earned |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other Segments [Member] | Other Insurance Product Line [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Direct Premiums Written |  |  |  |  |  |  |  |  | 29,800 | 44,300 | 62,800
Net premiums earned |  |  |  |  |  |  |  |  | $ 51,300 | $ 58,900 | $ 59,800

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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 (b),(c)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 + 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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