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Segment Information (Revenue by Segment) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Segment Reporting Information [Line Items]        
Total net premiums earned $ 614,277 $ 572,055 $ 1,810,941 $ 1,700,939
Other income 2,538 1,994 7,896 8,526
Net investment income 52,443 40,446 141,227 119,295
Net realized and unrealized investment (losses) gains (4,787) 6,798 (16,988) 7,487
Total investment revenues 47,656 47,244 124,239 126,782
Total revenues 664,471 621,293 1,943,076 1,836,247
Standard Commercial Lines [Member]        
Segment Reporting Information [Line Items]        
Other income 2,228 1,712 6,936 7,588
Total revenues 483,284 446,962 1,429,368 1,334,846
Standard Commercial Lines [Member] | Commercial Automobile [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 124,862 111,711 365,197 327,156
Standard Commercial Lines [Member] | Workers Compensation [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 78,784 77,580 237,628 236,366
Standard Commercial Lines [Member] | General Liability [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 154,974 141,059 457,805 422,546
Standard Commercial Lines [Member] | Commercial Property [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 83,056 78,151 245,544 232,594
Standard Commercial Lines [Member] | Business Owners' Policies [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 25,994 25,019 77,414 74,853
Standard Commercial Lines [Member] | Bonds Segment [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 8,778 7,420 25,247 20,904
Standard Commercial Lines [Member] | Other Commercial [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 4,608 4,310 13,597 12,839
Standard Personal Lines [Member]        
Segment Reporting Information [Line Items]        
Other income 310 282 959 938
Total revenues 77,467 72,883 228,049 216,412
Standard Personal Lines [Member] | Personal Automobile [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 42,772 38,612 125,024 113,225
Standard Personal Lines [Member] | Homeowners [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 32,293 32,215 96,717 97,382
Standard Personal Lines [Member] | Other Personal [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 2,092 1,774 5,349 4,867
Excess And Surplus Operations [Member]        
Segment Reporting Information [Line Items]        
Other income 0 0 1 0
Total revenues 56,064 54,204 161,420 158,207
Excess And Surplus Operations [Member] | Casualty Insurance Product Line [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned 42,179 40,090 120,098 117,056
Excess And Surplus Operations [Member] | Property Insurance Product Line [Member]        
Segment Reporting Information [Line Items]        
Total net premiums earned $ 13,885 $ 14,114 $ 41,321 $ 41,151