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Related Party Transactions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 29, 2023
Related Party Transaction [Line Items]
Equity ownership percentage of director of parent of rue insurance	100.00%
Direct premiums written	$ 5,319,457	$ 4,725,459	$ 4,068,518
Other liabilities	538,738	548,398
Contributions to the selective insurance group foundation	1,000	0	300
Investments	$ 9,651,297	8,693,729
Related Party Insurance Agency
Related Party Transaction [Line Items]
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
Commissions paid to Rue Insurance		2,900	2,700
Other liabilities		700
Costs and expenses, related party - BlackRock	$ 2,000	2,100	1,800
BlackRock Inc
Related Party Transaction [Line Items]
Other liabilities	$ 700	$ 600	800
Percentage common stock owned by related party		12.80%
Percentage of investments in related party funds to total invested assets	1.00%	1.00%
Related party transaction, purchases from related party	$ 5,700	$ 7,900	18,500
Sale of related party investments	10,800	2,800	32,300
Related party transactions, realized and unrealized gains (losses)	1,100	1,700	(6,900)
Investment income (loss), net	2,100	2,500	1,800
BlackRock Inc | Defined Benefit Pension Plan
Related Party Transaction [Line Items]
Related party transaction, purchases from related party	0	19,900	56,400
Sale of related party investments	18,100	35,100	65,700
Investment income (loss), net	(8,200)	9,300	(80,500)
Investments	$ 87,900	$ 114,200
Vanguard Group
Related Party Transaction [Line Items]
Percentage common stock owned by related party				10.24%
Percentage of investments in related party funds to total invested assets	1.00%	1.00%
Related party transactions, realized and unrealized gains (losses)	$ (1,000)	$ 100	(10,400)
Investment income (loss), net	1,200	700	4,700
Vanguard Group | Defined Benefit Pension Plan
Related Party Transaction [Line Items]
Related party transaction, purchases from related party	16,100	500	3,500
Sale of related party investments	$ 7,000	$ 32,900	$ 125,200

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The maximum percentage of the equity of an independent insurance agency owned by a daughter of a director of the Parent.

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