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Investment Income and Gains and Losses (Tables) | 12 Months Ended
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Dec. 31, 2025
Investments, Debt and Equity Securities [Abstract] |
Components of Net Investment Income | The components of net investment income were as follows:
YEARS ENDED DECEMBER 31 |  | 2025 |  |  | 2024 |  |  | 2023 |
(in millions) |  |  |  |  |  |  |  |  |
Fixed maturities |  | $ | 401.4 |  |  | $ | 324.4 |  |  | $ | 283.2 |
Limited partnerships |  |  | 27.1 |  |  |  | 20.6 |  |  |  | 30.0 |
Mortgage loans |  |  | 11.2 |  |  |  | 14.4 |  |  |  | 15.8 |
Equity securities |  |  | 3.6 |  |  |  | 3.2 |  |  |  | 6.4 |
Short-term and other investments |  |  | 26.4 |  |  |  | 25.2 |  |  |  | 10.6 |
Gross investment income |  |  | 469.7 |  |  |  | 387.8 |  |  |  | 346.0 |
Less: investment expenses |  |  | (15.3 | ) |  |  | (15.2 | ) |  |  | (13.9 | )
Net investment income |  | $ | 454.4 |  |  | $ | 372.6 |  |  | $ | 332.1 |
Schedule of Net Realized and Unrealized Gains (Losses) on Investments Including Impairments | Net realized and unrealized gains (losses) on investments, including impairments, were as follows:
YEARS ENDED DECEMBER 31 |  | 2025 |  |  | 2024 |  |  | 2023 |
(in millions) |  |  |  |  |  |  |  |  |
Fixed maturities |  | $ | (61.8 | ) |  | $ | (89.6 | ) |  | $ | (20.2 | )
Equity securities |  |  | 19.0 |  |  |  | 14.2 |  |  |  | (5.6 | )
Mortgage loans |  |  | (2.5 | ) |  |  | (0.5 | ) |  |  | (6.9 | )
Other investments |  |  | (0.7 | ) |  |  | 0.1 |  |  |  | 0.2 |
Net realized and unrealized investment losses |  | $ | (46.0 | ) |  | $ | (75.8 | ) |  | $ | (32.5 | )
Schedule of Pre-tax Net Realized and Unrealized Gains (Losses) on Equity Securities | The following table provides pre-tax net realized and unrealized gains (losses) on equity securities recognized in income:
YEARS ENDED DECEMBER 31 |  | 2025 |  |  | 2024 |  |  | 2023 |
(in millions) |  |  |  |  |  |  |  |  |
On securities still held |  | $ | 18.4 |  |  | $ | 14.2 |  |  | $ | 6.0 |
On securities sold during the period |  |  | 0.6 |  |  |  | — |  |  |  | (11.6 | )
Total net realized and unrealized gains (losses) on equity securities recognized in income |  | $ | 19.0 |  |  | $ | 14.2 |  |  | $ | (5.6 | )
Schedule of Allowance for Credit Losses | The following table provides rollforwards of the allowance for credit losses on mortgage loans:
YEARS ENDED DECEMBER 31 |  | 2025 |  |  | 2024 |
(in millions) |  |  |  |  |  |
Allowance for credit losses as of the beginning of the period |  | $ | 5.7 |  |  | $ | 10.0 |
Reductions for disposals |  |  | (0.5 | ) |  |  | (3.6 | )
Reductions for writedowns |  |  | — |  |  |  | (3.0 | )
Additional credit losses on investments for which an allowance was previously recognized |  |  | 2.9 |  |  |  | 2.3 |
Allowance for credit losses as of the end of the period |  | $ | 8.1 |  |  | $ | 5.7 |
Schedule of Realized Gain (Loss) | The proceeds from sales of available-for-sale fixed maturities, and the gross realized gains and gross realized losses on those sales, were as follows:
YEARS ENDED DECEMBER 31 |  | 2025 |  |  | 2024 |  |  | 2023 |
(in millions) |  |  |  |  |  |  |  |  |
Proceeds from sales |  | $ | 1,452.9 |  |  | $ | 1,844.1 |  |  | $ | 168.8 |
Gross gains |  |  | 2.4 |  |  |  | 2.5 |  |  |  | 1.0 |
Gross losses |  |  | 64.6 |  |  |  | 91.3 |  |  |  | 10.5 |

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