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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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