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Related Party Transactions (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Investments	$ 8,026,988	$ 7,505,599
Direct premiums written	3,656,537	3,204,512	$ 3,084,451
Contributions to the Selective Insurance Group Foundation	1,300	500	1,300
Costs and Expenses, Related Party - BlackRock	1,800	2,000	2,200
Due to Related Parties	$ 500	1,300
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%
Direct premiums written	$ 12,800	12,800	11,000
Commissions paid to Rue Insurance	2,000	1,800	2,000
Due to Related Parties, Current	$ 700	200
BlackRock Inc [Member]
Percentage Common Stock Owned by Related Party	11.40%
Related Party Transaction, Purchases from Related Party	$ 16,500	62,200	21,700
Sale of Related Party Investments	32,500
Net realized and unrealized gain (loss)	(600)	200	(5,700)
Investment Income (Loss), Net	900	400
Retirement Income Plan [Member] | BlackRock Inc [Member]
Investments	209,900	191,800
Related Party Transaction, Purchases from Related Party	18,000	56,700	19,700
Sale of Related Party Investments	18,100	44,900	44,100
Investment Income (Loss), Net	$ 18,200	$ 35,800	$ 36,700

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