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Segment Information (Income Before Federal Income Tax) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Segment Reporting Information [Line Items]    
Net investment income $ 37,419 $ 30,769
Net realized investment losses (1,045) (2,704)
Total investment income, before federal income tax 36,374 28,065
Tax on investment income 9,602 6,263
Total investment income, after federal income tax 26,772 21,802
Standard Commercial Lines [Member]    
Segment Reporting Information [Line Items]    
Underwriting gain, before federal income tax $ 42,546 $ 30,932
GAAP combined ratio 90.30% 92.30%
Statutory combined ratio 88.30% 89.70%
Standard Personal Lines [Member]    
Segment Reporting Information [Line Items]    
Underwriting gain, before federal income tax $ 5,106 $ 8,605
GAAP combined ratio 92.80% 87.70%
Statutory combined ratio 93.90% 90.60%
Excess And Surplus Operations [Member]    
Segment Reporting Information [Line Items]    
Underwriting gain, before federal income tax $ 1,570 $ 1,418
GAAP combined ratio 96.90% 97.10%
Statutory combined ratio 95.90% 98.40%