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Segment Information (Income Before Federal Income Tax) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Segment Reporting Information [Line Items]        
Net investment income $ 40,446 $ 33,375 $ 119,295 $ 95,326
Net realized investment gains 6,798 3,688 7,487 2,749
Total investment income, before federal income tax 47,244 37,063 126,782 98,075
Tax on investment income 13,236 9,752 34,572 24,290
Total investment income, after federal income tax 34,008 27,311 92,210 73,785
Standard Commercial Lines [Member]        
Segment Reporting Information [Line Items]        
Underwriting gain (loss), before federal income tax $ 35,329 $ 30,124 $ 112,634 $ 101,229
GAAP combined ratio 92.10% 92.90% 91.50% 91.80%
Statutory combined ratio 91.70% 92.00% 90.20% 90.10%
Standard Personal Lines [Member]        
Segment Reporting Information [Line Items]        
Underwriting gain (loss), before federal income tax $ 8,179 $ 4,271 $ 7,517 $ 19,001
GAAP combined ratio 88.70% 93.80% 96.50% 90.90%
Statutory combined ratio 86.40% 92.00% 95.20% 90.70%
Excess And Surplus Operations [Member]        
Segment Reporting Information [Line Items]        
Underwriting gain (loss), before federal income tax $ (11,063) $ (2,362) $ (8,174) $ (3,465)
GAAP combined ratio 120.40% 104.50% 105.20% 102.30%
Statutory combined ratio 120.10% 101.40% 104.60% 100.90%