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Segment Information (Revenue by Segment) (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2016 | Jun. 30, 2015 | Jun. 30, 2016 | Jun. 30, 2015
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | $ 531,932 | $ 490,309 | $ 1,054,390 | $ 966,432
Net investment income | 31,182 | 32,230 | 61,951 | 59,147
Net realized investment gains (losses) | 1,765 | (3,420) | (939) | 15,463
Total investment revenues | 32,947 | 28,810 | 61,012 | 74,610
Other income | 3,868 | 2,854 | 4,819 | 4,823
Total revenues | 568,747 | 521,973 | 1,120,221 | 1,045,865
Standard Commercial Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 414,844 | 379,768 | 818,423 | 746,970
Standard Commercial Lines [Member] | Commercial Automobile [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 98,896 | 88,658 | 194,315 | 175,013
Standard Commercial Lines [Member] | Workers Compensation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 75,251 | 70,954 | 151,251 | 139,431
Standard Commercial Lines [Member] | General Liability [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 129,283 | 119,207 | 257,368 | 234,178
Standard Commercial Lines [Member] | Commercial Property [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 73,591 | 66,549 | 143,769 | 131,112
Standard Commercial Lines [Member] | Business Owners' Policies [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 24,651 | 23,178 | 48,555 | 45,877
Standard Commercial Lines [Member] | Bonds Segment [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 5,665 | 5,106 | 11,129 | 10,106
Standard Commercial Lines [Member] | Other Commercial [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 3,940 | 3,553 | 7,779 | 7,021
Standard Commercial Lines [Member] | Miscellaneous Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 3,567 | 2,563 | 4,257 | 4,232
Standard Personal Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 71,125 | 72,362 | 141,590 | 145,141
Standard Personal Lines [Member] | Miscellaneous Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 301 | 291 | 561 | 591
Standard Personal Lines [Member] | Personal Automobile [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 35,881 | 36,740 | 71,661 | 73,750
Standard Personal Lines [Member] | Homeowners [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 33,411 | 33,731 | 66,311 | 67,452
Standard Personal Lines [Member] | Other Personal [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 1,532 | 1,600 | 3,057 | 3,348
Excess And Surplus Operations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 49,831 | 41,033 | 99,196 | 79,144
Excess And Surplus Operations [Member] | Commercial Automobile [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 2,663 | 1,776 | 5,061 | 3,506
Excess And Surplus Operations [Member] | General Liability [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 35,452 | 28,793 | 70,974 | 55,519
Excess And Surplus Operations [Member] | Commercial Property [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 11,716 | 10,464 | 23,160 | 20,119
Excess And Surplus Operations [Member] | Miscellaneous Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | $ 0 | $ 0 | $ 1 | $ 0

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