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Segment Information (Revenue From Continuing Operations by Segment) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2014 | Jun. 30, 2013 | Jun. 30, 2014 | Jun. 30, 2013
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | $ 463,625 | $ 426,252 | $ 920,120 | $ 847,192
Other income | 1,911 | 3,536 | 11,735 | 6,320
Net investment income earned | 36,774 | 34,003 | 72,308 | 66,873
Net realized investment gains | 4,539 | 5,154 | 11,757 | 8,509
Total investment revenues | 41,313 | 39,157 | 84,065 | 75,382
Total all segments | 506,846 | 468,937 | 1,015,912 | 928,822
Total revenues from continuing operations | 506,849 | 468,945 | 1,015,920 | 928,894
Commercial Automobile [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 83,472 | 76,706 | 165,688 | 151,053
Workers Compensation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 68,992 | 64,855 | 138,405 | 130,939
General Liability [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 111,591 | 99,766 | 220,409 | 197,469
Commercial Property [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 61,226 | 54,937 | 121,412 | 108,352
Business Owners' Policies [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 21,279 | 18,625 | 42,148 | 37,165
Bonds Segment [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 4,734 | 4,775 | 9,490 | 9,539
Other Commercial [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 3,213 | 2,993 | 6,396 | 5,985
Total Standard Commercial Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 354,507 | 322,657 | 703,948 | 640,502
Personal Automobile [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 38,022 | 38,526 | 76,248 | 76,919
Homeowners [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 33,576 | 31,702 | 66,874 | 62,837
Other Personal [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 2,946 | 3,320 | 6,240 | 6,828
Total Standard Personal Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 74,544 | 73,548 | 149,362 | 146,584
Total Standard Insurance Operations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 429,051 | 396,205 | 853,310 | 787,086
Total Standard insurance Operations revenues | 430,959 | 399,733 | 865,037 | 793,334
Miscellaneous Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Other income | 1,908 | 3,528 | 11,727 | 6,248
Excess And Surplus Operations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 34,574 | 30,047 | 66,810 | 60,106
Other Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Other income | $ 3 | $ 8 | $ 8 | $ 72

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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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