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Statutory Financial Information, Capital Requirements, and Restrictions on Dividends and Transfers of Funds - Schedule of Insurance Subsidiaries Statutory Surplus Data (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Statutory Accounting Practices [Line Items] |  |  |
Unassigned Surplus | $ 2,337.2 | $ 2,176.7 |
Statutory Surplus | 2,902.8 | 2,742.3 |
Statutory Net Income | 178.3 | 318.8 | $ 264.4
Selective Insurace Company of America (SICA) |  |  |
Statutory Accounting Practices [Line Items] |  |  |
Unassigned Surplus | 832.3 | 773.6 |
Statutory Surplus | 997.5 | 938.8 |
Statutory Net Income | 76.8 | 120.4 | 103.1
Selective Way Insurance Company ("SWIC") |  |  |
Statutory Accounting Practices [Line Items] |  |  |
Unassigned Surplus | 537.1 | 513.5 |
Statutory Surplus | 593.1 | 569.3 |
Statutory Net Income | 36.3 | 64.1 | 69.6
Selective Insurance Company of South Carolina (SICSC) |  |  |
Statutory Accounting Practices [Line Items] |  |  |
Unassigned Surplus | 219.9 | 201.3 |
Statutory Surplus | 254.1 | 235.6 |
Statutory Net Income | 13.3 | 26.0 | 17.0
Selective Insurance Company of the Southeast (SICSE) |  |  |
Statutory Accounting Practices [Line Items] |  |  |
Unassigned Surplus | 171.9 | 155.1 |
Statutory Surplus | 199.5 | 182.7 |
Statutory Net Income | 11.2 | 21.6 | 14.2
Selective Insurance Company of New York (SICNY) |  |  |
Statutory Accounting Practices [Line Items] |  |  |
Unassigned Surplus | 162.0 | 152.4 |
Statutory Surplus | 189.7 | 180.1 |
Statutory Net Income | 7.1 | 16.4 | 13.2
Selective Insurance Company of New England ("SICNE") |  |  |
Statutory Accounting Practices [Line Items] |  |  |
Unassigned Surplus | 49.2 | 45.5 |
Statutory Surplus | 80.3 | 76.7 |
Statutory Net Income | 3.3 | 8.3 | 5.1
Selective Auto Insurance Company of New Jersey ("SAICNJ") |  |  |
Statutory Accounting Practices [Line Items] |  |  |
Unassigned Surplus | 127.5 | 119.3 |
Statutory Surplus | 172.4 | 164.2 |
Statutory Net Income | 7.4 | 16.2 | 12.7
Mesa Underwriters Specialty Insurance Company ("MUSIC") |  |  |
Statutory Accounting Practices [Line Items] |  |  |
Unassigned Surplus | 72.3 | 64.3 |
Statutory Surplus | 141.8 | 133.7 |
Statutory Net Income | 8.7 | 14.4 | 9.7
Selective Casualty Insurance Company ("SCIC") |  |  |
Statutory Accounting Practices [Line Items] |  |  |
Unassigned Surplus | 117.2 | 107.8 |
Statutory Surplus | 193.6 | 184.3 |
Statutory Net Income | 10.7 | 24.0 | 14.0
Selective Fire and Casualty Insurance Company ("SFCIC") |  |  |
Statutory Accounting Practices [Line Items] |  |  |
Unassigned Surplus | 47.8 | 43.9 |
Statutory Surplus | 80.8 | 76.9 |
Statutory Net Income | $ 3.5 | $ 7.4 | $ 5.8

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