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Related Party Transactions (Narrative) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Equity ownership percentage of director of parent of general independent retail insurance agency 10.00%    
Direct premiums written $ 2,733,459 $ 2,577,259 $ 2,403,519
Due to Related Parties, Current 600 700  
Contributions to the Selective Insurance Group Foundation $ 100 0 1,000
Percentage Common Stock Owned by Related Party 12.80%    
Costs and Expenses, Related Party $ 2,000 400  
Due to Related Parties 500 400  
Related Party Insurance Agency [Member]      
Direct premiums written 11,100 10,400 9,600
Commissions paid to Rue Insurance $ 2,300 $ 2,100 $ 1,700