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Segment Information (Revenue by Segment) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Segment Reporting Information [Line Items]        
Total net premiums earned $ 604,836 $ 568,030 $ 1,196,664 $ 1,128,884
Other income 3,179 3,291 5,358 6,532
Net investment income 45,553 41,430 88,784 78,849
Net realized and unrealized investment (losses) gains (1,652) 1,734 (12,201) 689
Total investment revenues 43,901 43,164 76,583 79,538
Total revenues 651,916 614,485 1,278,605 1,214,954
Standard Commercial Lines [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 478,894 446,610 946,084 887,884
Other income 2,882 3,016 4,708 5,876
Standard Commercial Lines [Member] | Commercial Automobile [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 122,104 108,316 240,335 215,445
Standard Commercial Lines [Member] | Workers Compensation [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 80,021 79,460 158,844 158,786
Standard Commercial Lines [Member] | General Liability [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 153,002 141,503 302,831 281,487
Standard Commercial Lines [Member] | Commercial Property [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 82,162 78,052 162,488 154,443
Standard Commercial Lines [Member] | Business Owners' Policies [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 25,829 24,989 51,420 49,834
Standard Commercial Lines [Member] | Bonds Segment [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 8,335 6,986 16,469 13,484
Standard Commercial Lines [Member] | Other Commercial [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 4,559 4,288 8,989 8,529
Standard Personal Lines [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 75,974 71,947 150,582 143,529
Other income 297 275 649 656
Standard Personal Lines [Member] | Personal Automobile [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 41,810 37,663 82,252 74,613
Standard Personal Lines [Member] | Homeowners [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 32,223 32,467 64,424 65,167
Standard Personal Lines [Member] | Other Personal [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 1,644 1,542 3,257 3,093
Excess And Surplus Operations [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 53,147 52,764 105,356 104,003
Other income 0 0 1 0
Excess And Surplus Operations [Member] | General Liability [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 39,379 39,054 77,919 76,966
Excess And Surplus Operations [Member] | Commercial Property [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned $ 13,768 $ 13,710 $ 27,436 $ 27,037