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Segment Information (Revenue From Continuing Operations by Segment) (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2018
 [1]
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
 [1]
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Revenue from External Customer [Line Items]                      
Net premiums earned $ 625,288 $ 614,277 $ 604,836 $ 591,828 $ 590,088 $ 572,055 $ 568,030 $ 560,854 $ 2,436,229 $ 2,291,027 $ 2,149,572
Other income 1,542 2,538 3,179 2,179 2,190 1,994 3,291 3,241 9,438 10,716 8,881
Net investment income 54,109 52,443 45,553 43,231 42,587 40,446 41,430 37,419 195,336 161,882 130,754
Total net realized and unrealized (losses) gains (37,935) [2] (4,787) [2] (1,652) [2] (10,549) [2] (1,128) [2] 6,798 [2] 1,734 [2] (1,045) [2] (54,923) 6,359 (4,937)
Total investment revenues                 140,413 168,241 125,817
Total revenues $ 643,004 $ 664,471 $ 651,916 $ 626,689 $ 633,737 $ 621,293 $ 614,485 $ 600,469 2,586,080 2,469,984 2,284,270
Standard Commercial Lines [Member]                      
Revenue from External Customer [Line Items]                      
Other income                 8,180 9,488 7,782
Total revenues                 1,920,402 1,797,987 1,673,265
Standard Commercial Lines [Member] | Commercial Automobile [Member]                      
Revenue from External Customer [Line Items]                      
Net premiums earned                 493,093 442,818 398,942
Standard Commercial Lines [Member] | Workers Compensation [Member]                      
Revenue from External Customer [Line Items]                      
Net premiums earned                 317,616 317,982 308,233
Standard Commercial Lines [Member] | General Liability [Member]                      
Revenue from External Customer [Line Items]                      
Net premiums earned                 616,187 569,217 527,859
Standard Commercial Lines [Member] | Commercial Property [Member]                      
Revenue from External Customer [Line Items]                      
Net premiums earned                 329,660 311,932 293,438
Standard Commercial Lines [Member] | Business Owners' Policies [Member]                      
Revenue from External Customer [Line Items]                      
Net premiums earned                 103,412 100,266 97,754
Standard Commercial Lines [Member] | Bonds Segment [Member]                      
Revenue from External Customer [Line Items]                      
Net premiums earned                 33,991 29,086 23,227
Standard Commercial Lines [Member] | Other Commercial [Member]                      
Revenue from External Customer [Line Items]                      
Net premiums earned                 18,263 17,198 16,030
Standard Personal Lines [Member]                      
Revenue from External Customer [Line Items]                      
Other income                 1,257 1,228 1,098
Total revenues                 305,698 290,929 281,705
Standard Personal Lines [Member] | Personal Automobile [Member]                      
Revenue from External Customer [Line Items]                      
Net premiums earned                 168,250 153,147 142,876
Standard Personal Lines [Member] | Homeowners [Member]                      
Revenue from External Customer [Line Items]                      
Net premiums earned                 128,961 129,699 130,973
Standard Personal Lines [Member] | Other Personal [Member]                      
Revenue from External Customer [Line Items]                      
Net premiums earned                 7,230 6,855 6,758
Excess And Surplus Operations [Member]                      
Revenue from External Customer [Line Items]                      
Other income                 1 0 1
Total revenues                 219,567 212,827 203,483
Excess And Surplus Operations [Member] | Casualty Insurance Product Line [Member]                      
Revenue from External Customer [Line Items]                      
Net premiums earned                 164,313 157,366 151,638
Excess And Surplus Operations [Member] | Property Insurance Product Line [Member]                      
Revenue from External Customer [Line Items]                      
Net premiums earned                 $ 55,253 $ 55,461 $ 51,844
[1] Results for the fourth quarter of 2017 include the impact of the $20.2 million write off of deferred tax assets required with the implementation of Tax Reform. See Note 13. "Federal Income Taxes" above for additional information.
[2] Effective January 1, 2018, changes in unrealized gains and losses on our equity portfolio are recognized in income through "Net unrealized losses on equity securities" on our Consolidated Statements of Income, as a result of our adoption of ASU 2016-01. See Note 3. "Adoption of Accounting Pronouncements" above.