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Segment Information (Revenue From Continuing Operations by Segment) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2013 | Sep. 30, 2012
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | $ 437,568 | $ 406,225 | $ 1,284,760 | $ 1,177,266
Other income | 3,357 | 1,085 | 9,677 | 7,129
Net investment income | 32,457 | 30,650 | 99,330 | 97,284
Net realized investment gains | 13,431 | (1,088) | 21,940 | 3,448
Total investment revenues | 45,888 | 29,562 | 121,270 | 100,732
Total all segments | 486,798 | 436,796 | 1,415,620 | 1,284,902
Total revenues | 486,813 | 436,872 | 1,415,707 | 1,285,127
Commercial Automobile [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 79,138 | 72,758 | 230,191 | 214,782
Workers Compensation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 66,510 | 65,592 | 197,449 | 198,064
General Liability [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 100,925 | 93,763 | 298,394 | 276,538
Commercial Property [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 57,004 | 52,197 | 165,356 | 151,945
Business Owners' Policies [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 19,629 | 17,749 | 56,794 | 51,872
Bonds Segment [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 4,705 | 4,713 | 14,244 | 14,076
Other Commercial [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 3,051 | 2,921 | 9,036 | 9,202
Total standard Commercial Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 330,962 | 309,693 | 971,464 | 916,479
Personal Automobile [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 38,513 | 38,295 | 115,432 | 113,648
Homeowners [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 32,374 | 29,919 | 95,211 | 86,685
Other Personal [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 3,827 | 3,591 | 10,655 | 10,037
Total standard Personal Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 74,714 | 71,805 | 221,298 | 210,370
Total Standard Insurance Operations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 405,676 | 381,498 | 1,192,762 | 1,126,849
Total insurance operations revenues | 409,018 | 382,507 | 1,202,352 | 1,133,753
Miscellaneous Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Other income | 3,342 | 1,009 | 9,590 | 6,904
Excess And Surplus Operations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 31,892 | 24,727 | 91,998 | 50,417
Other Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Other income | $ 15 | $ 76 | $ 87 | $ 225

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- Definition Amount of total revenue for the reportable segments, including revenue from external customers, transactions with other operating segments of the same entity, and (gross) interest revenue. All considered to be "operating revenue" for purposes of the taxonomy. + References No definition available. + Details
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- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of property and casualty 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 SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1 -Article 7 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486662&loc=d3e27175-158546 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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