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Investments Fixed Income Securities by Type (Details) - USD ($) $ in Millions | Jun. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Schedule of Fixed Income Securities [Table] [Line Items] |  |
Fixed income securities (amortized cost) | $ 12,210.9 |
Fixed income securities (estimated fair value) | 12,357.2 | $ 12,091.5
Debt Securities, Available-for-sale, Allowance for Credit Loss | 1.6 | 1.6
U.S. & Canadian Governments [Member] |  |
Schedule of Fixed Income Securities [Table] [Line Items] |  |
Fixed income securities (amortized cost) | 1,839.2 | 1,950.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 9.8 | 2.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 30.7 | 61.3
Fixed income securities (estimated fair value) | 1,818.3 | 1,891.9
Municipal [Member] |  |
Schedule of Fixed Income Securities [Table] [Line Items] |  |
Fixed income securities (amortized cost) | 184.2 | 319.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 0.0 | 0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0.3 | 1.6
Fixed income securities (estimated fair value) | 183.8 | 317.9
Corporate [Member] |  |
Schedule of Fixed Income Securities [Table] [Line Items] |  |
Fixed income securities (amortized cost) | 10,187.4 | 9,905.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 198.0 | 76.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 30.4 | 100.8
Fixed income securities (estimated fair value) | 10,355.0 | 9,881.7
Fixed Income Securities [Member] |  |
Schedule of Fixed Income Securities [Table] [Line Items] |  |
Fixed income securities (amortized cost) | 12,210.9 | 12,175.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 207.9 | 79.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 61.6 | 163.8
Fixed income securities (estimated fair value) | $ 12,357.2 | $ 12,091.5

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