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Segment Information (Revenue by Segment) (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2019 | Mar. 31, 2018
Segment Reporting Information [Line Items] |  |
Total net premiums earned | $ 632,573 | $ 591,828
Other income | 2,320 | 2,179
Net investment income | 50,618 | 43,231
Net realized and unrealized investment gains (losses) | 13,451 | (10,549)
Total investment revenues | 64,069 | 32,682
Total revenues | 698,962 | 626,689
Standard Commercial Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Other income | 2,044 | 1,826
Total revenues | 499,228 | 467,190
Standard Commercial Lines [Member] | Commercial Automobile [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 131,186 | 118,231
Standard Commercial Lines [Member] | Workers Compensation [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 78,715 | 78,823
Standard Commercial Lines [Member] | General Liability [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 161,525 | 149,829
Standard Commercial Lines [Member] | Commercial Property [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 86,067 | 80,326
Standard Commercial Lines [Member] | Business Owners' Policies [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 26,081 | 25,591
Standard Commercial Lines [Member] | Bonds Segment [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 8,904 | 8,134
Standard Commercial Lines [Member] | Other Commercial [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 4,706 | 4,430
Standard Personal Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Other income | 276 | 352
Total revenues | 77,583 | 74,608
Standard Personal Lines [Member] | Personal Automobile [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 43,163 | 40,442
Standard Personal Lines [Member] | Homeowners [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 32,130 | 32,201
Standard Personal Lines [Member] | Other Personal [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 2,014 | 1,613
Excess And Surplus Operations [Member] |  |
Segment Reporting Information [Line Items] |  |
Other income | 0 | 1
Total revenues | 58,082 | 52,209
Excess And Surplus Operations [Member] | Casualty Insurance Product Line [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 44,528 | 38,540
Excess And Surplus Operations [Member] | Property Insurance Product Line [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | $ 13,554 | $ 13,668

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