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Segment Information (Revenue by Segment) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | $ 572,055 | $ 542,429 | $ 1,700,939 | $ 1,596,819
Net investment income | 40,446 | 33,375 | 119,295 | 95,326
Net realized investment gains | 6,798 | 3,688 | 7,487 | 2,749
Total investment revenues | 47,244 | 37,063 | 126,782 | 98,075
Other income | 1,994 | 2,199 | 8,526 | 7,018
Total revenues | 621,293 | 581,691 | 1,836,247 | 1,701,912
Standard Commercial Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 446,962 | 423,511 | 1,334,846 | 1,241,934
Standard Commercial Lines [Member] | Commercial Automobile [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 111,711 | 100,612 | 327,156 | 294,927
Standard Commercial Lines [Member] | Workers Compensation [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 77,580 | 78,596 | 236,366 | 229,847
Standard Commercial Lines [Member] | General Liability [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 141,059 | 133,981 | 422,546 | 391,349
Standard Commercial Lines [Member] | Commercial Property [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 78,151 | 74,052 | 232,594 | 217,821
Standard Commercial Lines [Member] | Business Owners' Policies [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 25,019 | 24,461 | 74,853 | 73,016
Standard Commercial Lines [Member] | Bonds Segment [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 7,420 | 5,795 | 20,904 | 16,924
Standard Commercial Lines [Member] | Other Commercial [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 4,310 | 4,089 | 12,839 | 11,868
Standard Commercial Lines [Member] | Miscellaneous Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 1,712 | 1,925 | 7,588 | 6,182
Standard Personal Lines [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 72,883 | 68,965 | 216,412 | 210,555
Standard Personal Lines [Member] | Miscellaneous Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 282 | 275 | 938 | 836
Standard Personal Lines [Member] | Personal Automobile [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 38,612 | 34,865 | 113,225 | 106,526
Standard Personal Lines [Member] | Homeowners [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 32,215 | 32,031 | 97,382 | 98,342
Standard Personal Lines [Member] | Other Personal [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 1,774 | 1,794 | 4,867 | 4,851
Excess And Surplus Operations [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 54,204 | 52,152 | 158,207 | 151,348
Excess And Surplus Operations [Member] | General Liability [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 40,090 | 38,991 | 117,056 | 112,787
Excess And Surplus Operations [Member] | Commercial Property [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | 14,114 | 13,162 | 41,151 | 38,561
Excess And Surplus Operations [Member] | Miscellaneous Income [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Total net premiums earned | $ 0 | $ (1) | $ 0 | $ 0

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