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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 |

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