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Segment Information (Income Before Federal Income Tax) (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Segment Reporting Information [Line Items]        
Net investment income $ 41,430 $ 31,182 $ 78,849 $ 61,951
Net realized investment gains (losses) 1,734 1,765 689 (939)
Total investment income, before federal income tax 43,164 32,947 79,538 61,012
Tax on investment income 11,734 8,275 21,336 14,538
Total investment income, after federal income tax 31,430 24,672 58,202 46,474
Standard Commercial Lines [Member]        
Segment Reporting Information [Line Items]        
Underwriting gain (loss), before federal income tax $ 34,759 $ 40,173 $ 77,305 $ 71,105
GAAP combined ratio 92.20% 90.20% 91.20% 91.30%
Statutory combined ratio 90.60% 88.60% 89.50% 89.10%
Standard Personal Lines [Member]        
Segment Reporting Information [Line Items]        
Underwriting gain (loss), before federal income tax $ (5,768) $ 6,125 $ (662) $ 14,730
GAAP combined ratio 108.00% 91.40% 100.50% 89.60%
Statutory combined ratio 105.90% 89.90% 99.80% 90.10%
Excess And Surplus Operations [Member]        
Segment Reporting Information [Line Items]        
Underwriting gain (loss), before federal income tax $ 1,319 $ (2,521) $ 2,889 $ (1,103)
GAAP combined ratio 97.50% 105.10% 97.20% 101.10%
Statutory combined ratio 97.00% 102.70% 96.50% 100.60%