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Related Party Transactions (Narrative) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Jan. 31, 2023
Equity Ownership Percentage Of Director Of Parent Of Rue Insurance 100.00%      
Direct premiums written $ 4,725,459 $ 4,068,518 $ 3,656,537  
Contributions to the Selective Insurance Group Foundation   300 1,300  
Other liabilities 548,398 517,234    
Investments $ 8,693,729 7,837,469    
BlackRock Inc [Member]        
Percentage Common Stock Owned by Related Party 12.80%      
Other liabilities $ 600 $ 800 500  
Percentage of investments in related party funds to total invested assets 1.00% 1.00%    
Related Party Transaction, Purchases from Related Party $ 7,900 $ 18,500 16,500  
Sale of Related Party Investments 2,800 32,300 32,500  
Related Party Transactions, Realized and unrealized gains (losses) 1,700 6,900 600  
Investment Income (Loss), Net $ (2,500) (1,800) (900)  
Related Party Insurance Agency [Member]        
Equity ownership percentage of director of parent of general independent retail insurance agency 10.00%      
Equity Ownership Percentage Of Son Of Director Of Parent 10.00%      
Equity Ownership Percentage Of Daughter Of Director Of Parent 10.00%      
Direct premiums written $ 15,700 14,300 12,800  
Commissions paid to Rue Insurance 2,900 2,700 2,000  
Costs and Expenses, Related Party - BlackRock 2,100 1,800 1,800  
Other liabilities 700 $ 700    
Vanguard Group [Member]        
Percentage Common Stock Owned by Related Party       10.03%
Percentage of investments in related party funds to total invested assets   1.00%    
Related Party Transactions, Realized and unrealized gains (losses) 100 $ 10,400 17,700  
Investment Income (Loss), Net (700) (4,700) (7,500)  
Retirement Income Plan [Member] | BlackRock Inc [Member]        
Related Party Transaction, Purchases from Related Party 19,900 56,400 18,000  
Sale of Related Party Investments 35,100 65,700 18,100  
Investments 114,200 120,100    
Investment Income (Loss), Net 9,300 (80,500) 18,200  
Retirement Income Plan [Member] | Vanguard Group [Member]        
Related Party Transaction, Purchases from Related Party 500 3,500 19,300  
Sale of Related Party Investments $ 32,900 $ 125,200 $ 23,600