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Segment Information (Revenue by Segment) (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Segment Reporting Information [Line Items] |  |
Total net premiums earned | $ 591,828 | $ 560,854
Other income | 2,179 | 3,241
Net investment income | 43,231 | 37,419
Net realized and unrealized investment losses | (10,549) | (1,045)
Total investment revenues | 32,682 | 36,374
Total revenues | 626,689 | 600,469
Standard Commercial Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 467,190 | 441,274
Other income | 1,826 | 2,860
Standard Commercial Lines [Member] | Commercial Automobile [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 118,231 | 107,129
Standard Commercial Lines [Member] | Workers Compensation [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 78,823 | 79,326
Standard Commercial Lines [Member] | General Liability [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 149,829 | 139,984
Standard Commercial Lines [Member] | Commercial Property [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 80,326 | 76,391
Standard Commercial Lines [Member] | Business Owners' Policies [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 25,591 | 24,845
Standard Commercial Lines [Member] | Bonds Segment [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 8,134 | 6,498
Standard Commercial Lines [Member] | Other Commercial [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 4,430 | 4,241
Standard Personal Lines [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 74,608 | 71,582
Other income | 352 | 381
Standard Personal Lines [Member] | Personal Automobile [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 40,442 | 36,950
Standard Personal Lines [Member] | Homeowners [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 32,201 | 32,700
Standard Personal Lines [Member] | Other Personal [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 1,613 | 1,551
Excess And Surplus Operations [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 52,209 | 51,239
Other income | 1 | 0
Excess And Surplus Operations [Member] | General Liability [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | 38,540 | 37,912
Excess And Surplus Operations [Member] | Commercial Property [Member] |  |
Segment Reporting Information [Line Items] |  |
Total net premiums earned | $ 13,668 | $ 13,327

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- Definition This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (c)-(e) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c),9(a)) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27405-111563 + Details
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