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Segment Information (Revenue by Segment) (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2016 | Mar. 31, 2015
Segment Reporting Information [Line Items] |  |
Total net premiums earned | $ 522,458 | $ 476,123
Net investment income | 30,769 | 26,917
Net realized investment (losses) gains | (2,704) | 18,883
Total investment revenues | 28,065 | 45,800
Other income | 951 | 1,969
Total revenues | 551,474 | 523,892
Standard Commercial Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 403,579 | 367,202
Standard Commercial Lines [Member] | Commercial Automobile [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 95,419 | 86,355
Standard Commercial Lines [Member] | Workers Compensation [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 76,000 | 68,477
Standard Commercial Lines [Member] | General Liability [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 128,085 | 114,971
Standard Commercial Lines [Member] | Commercial Property [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 70,178 | 64,563
Standard Commercial Lines [Member] | Business Owners' Policies [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 23,904 | 22,699
Standard Commercial Lines [Member] | Bonds Segment [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 5,464 | 5,000
Standard Commercial Lines [Member] | Other Commercial [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 3,839 | 3,468
Standard Commercial Lines [Member] | Miscellaneous Income [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 690 | 1,669
Standard Personal Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 70,465 | 72,779
Standard Personal Lines [Member] | Miscellaneous Income [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 260 | 300
Standard Personal Lines [Member] | Personal Automobile [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 35,780 | 37,010
Standard Personal Lines [Member] | Homeowners [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 32,900 | 33,721
Standard Personal Lines [Member] | Other Personal [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 1,525 | 1,748
Excess And Surplus Operations [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 49,365 | 38,111
Excess And Surplus Operations [Member] | Commercial Automobile [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 2,398 | 1,730
Excess And Surplus Operations [Member] | General Liability [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 35,522 | 26,726
Excess And Surplus Operations [Member] | Commercial Property [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 11,444 | 9,655
Excess And Surplus Operations [Member] | Miscellaneous Income [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | $ 1 | $ 0

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