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Segment Information (Revenue by Segment) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | $ 507,390 | $ 462,639 | $ 1,473,822 | $ 1,382,759
Net investment income earned | 32,061 | 34,292 | 91,208 | 106,600
Net realized investment gains | 308 | 15,231 | 15,771 | 26,988
Total investment revenues | 32,369 | 49,523 | 106,979 | 133,588
Other income | 698 | 3,196 | 5,521 | 14,931
Total revenues | 540,457 | 515,358 | 1,586,322 | 1,531,278
Operating Segments [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total revenues | 540,457 | 515,358 | 1,586,322 | 1,531,270
Other Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Other income | 0 | 0 | 0 | 8
Standard Commercial Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 389,990 | 354,973 | 1,136,960 | 1,069,406
Standard Commercial Lines [Member] | Commercial Automobile [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 90,758 | 83,536 | 265,771 | 249,224
Standard Commercial Lines [Member] | Workers Compensation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 74,560 | 66,732 | 213,991 | 205,137
Standard Commercial Lines [Member] | General Liability [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 123,252 | 110,894 | 357,430 | 331,303
Standard Commercial Lines [Member] | Commercial Property [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 68,587 | 61,304 | 199,699 | 182,716
Standard Commercial Lines [Member] | Business Owners' Policies [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 23,726 | 21,649 | 69,603 | 63,797
Standard Commercial Lines [Member] | Bonds Segment [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 5,031 | 4,791 | 15,137 | 14,281
Standard Commercial Lines [Member] | Other Commercial [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 3,628 | 3,237 | 10,649 | 9,633
Standard Commercial Lines [Member] | Miscellaneous Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 448 | 2,830 | 4,680 | 13,315
Standard Personal Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 72,338 | 74,743 | 217,479 | 225,347
Standard Personal Lines [Member] | Miscellaneous Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 250 | 366 | 841 | 1,608
Standard Personal Lines [Member] | Personal Automobile [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 36,623 | 37,695 | 110,373 | 113,943
Standard Personal Lines [Member] | Homeowners [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 33,670 | 33,957 | 101,122 | 100,831
Standard Personal Lines [Member] | Other Personal [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 1,795 | 2,725 | 5,143 | 8,965
Excess And Surplus Operations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 45,760 | 36,119 | 124,904 | 102,929
Excess And Surplus Operations [Member] | Commercial Automobile [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 2,056 | 1,412 | 5,562 | 3,859
Excess And Surplus Operations [Member] | General Liability [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 32,395 | 24,659 | 87,914 | 70,526
Excess And Surplus Operations [Member] | Commercial Property [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | $ 11,309 | $ 10,048 | $ 31,428 | $ 28,544

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