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Segment Information (Income From Continuing Operations, Before Federal Income Tax) (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Net investment income $ 42,587 $ 40,446 $ 41,430 $ 37,419 $ 35,428 $ 33,375 $ 31,182 $ 30,769 $ 161,882 $ 130,754 $ 121,316
Realized investment gains (losses)                 6,359 (4,937) 13,171
Total investment revenues                 168,241 125,817 134,487
Tax on investment income                 45,588 30,621 32,090
Total investment income, after federal income tax                 122,653 95,196 102,397
Standard Commercial Lines [Member]                      
Underwriting gain (loss), before federal income tax                 149,514 146,435 164,496
Underwriting gain (loss), after federal income tax                 $ 97,184 $ 95,183 $ 106,923
Combined Ratio                 91.60% 91.20% 89.20%
Standard Personal Lines [Member]                      
Underwriting gain (loss), before federal income tax                 $ 11,104 $ 12,419 $ 1,336
Underwriting gain (loss), after federal income tax                 $ 7,217 $ 8,072 $ 868
Combined Ratio                 96.20% 95.60% 99.50%
Excess And Surplus Operations [Member]                      
Underwriting gain (loss), before federal income tax                 $ (6,282) $ (6,921) $ (16,803)
Underwriting gain (loss), after federal income tax                 $ (4,083) $ (4,499) $ (10,922)
Combined Ratio                 103.00% 103.40% 109.80%