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Segment Information (Revenue From Continuing Operations by Segment) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned | $ 406,853 | $ 406,225 | $ 392,212 | $ 378,829 | $ 373,427 | $ 358,963 | $ 355,580 | $ 351,343 | $ 1,584,119 |  | $ 1,439,313 |  | $ 1,416,598 |
Other income |  |  |  |  |  |  |  |  | 9,118 |  | 8,479 |  | 9,398 |
Net investment income | 34,593 | 30,650 | 34,006 | 32,628 | 28,839 | 35,786 | 39,345 | 43,473 | 131,877 |  | 147,443 |  | 145,708 |
Net realized (gains) losses |  |  |  |  |  |  |  |  | 8,988 |  | 2,240 |  | (7,083) |
Total investment revenues |  |  |  |  |  |  |  |  | 140,865 |  | 149,683 |  | 138,625 |
Total all segments |  |  |  |  |  |  |  |  | 1,733,811 |  | 1,597,065 |  | 1,564,453 |
Total revenues |  |  |  |  |  |  |  |  | 1,734,102 |  | 1,597,475 |  | 1,564,621 |
Commercial Automobile [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned |  |  |  |  |  |  |  |  | 288,010 |  | 279,610 |  | 291,495 |
Workers' Compensation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned |  |  |  |  |  |  |  |  | 262,108 |  | 259,354 |  | 252,441 |
General Liability [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned |  |  |  |  |  |  |  |  | 373,381 |  | 344,682 |  | 336,475 |
Commercial Property [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned |  |  |  |  |  |  |  |  | 202,340 |  | 192,989 |  | 199,252 |
Business Owners' Policies [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned |  |  |  |  |  |  |  |  | 68,462 |  | 66,225 |  | 65,260 |
Bonds Segment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned |  |  |  |  |  |  |  |  | 18,891 |  | 18,910 |  | 19,243 |
Other Commercial [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned |  |  |  |  |  |  |  |  | 12,143 |  | 9,177 |  | 10,116 |
Total standard Commercial Lines [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned |  |  |  |  |  |  |  |  | 1,225,335 |  | 1,170,947 |  | 1,174,282 |
Personal Automobile [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned |  |  |  |  |  |  |  |  | 152,142 |  | 148,824 |  | 141,962 |
Homeowners [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned |  |  |  |  |  |  |  |  | 113,850 |  | 102,764 |  | 87,862 |
Other Personal [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned |  |  |  |  |  |  |  |  | 13,563 |  | 12,864 |  | 12,492 |
Total standard Personal Lines [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned |  |  |  |  |  |  |  |  | 279,555 |  | 264,452 |  | 242,316 |
Total Standard Insurance Operations [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned |  |  |  |  |  |  |  |  | 1,504,890 |  | 1,435,399 |  | 1,416,598 |
Total Insurance Operations revenues |  |  |  |  |  |  |  |  | 1,513,717 |  | 1,443,468 |  | 1,425,828 |
Miscellaneous Income [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other income |  |  |  |  |  |  |  |  | 8,827 | [1] | 8,069 | [2] | 9,230 | [3]
Excess And Surplus Operations [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Net Premiums Earned |  |  |  |  |  |  |  |  | 79,229 |  | 3,914 |  | 0 |
Other Income [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other income |  |  |  |  |  |  |  |  | $ 291 |  | $ 410 |  | $ 168 |
[1] | 4 In addition to amounts related to the Standard and E&S Insurance Operations segments, “Other income” and “Other expenses” on the Consolidated Statements of Income includes holding company income and expense amounts of $291 and $20,642, respectively.
[2] | 4 In addition to amounts related to the Standard and E&S Insurance Operations segments, “Other income” and “Other expenses” on the Consolidated Statements of Income includes holding company income and expense amounts of $410 and $16,850, respectively.
[3] | 3 In addition to amounts related to the Insurance Operations segment, “Other income” and “Other expenses” on the Consolidated Statements of Income includes holding company income and expense amounts of $168 and $16,054, respectively.

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- Definition Premiums recognized as revenue in the period earned on all property and casualty insurance and reinsurance contracts after subtracting any amounts ceded to another insurer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 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),(c) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 27 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: 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 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1 -Article 7 + Details
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