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Investments - Net Investment Income (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Net Investment Income [Line Items] |  |  |  |
Investment income (loss) | $ 102.6 | $ 98.7 | $ 211.1 | $ 207.0
Investment expenses | 6.7 | 5.7 | 14.0 | 13.6
Net Investment Income | 95.9 | 93.0 | 197.1 | 193.4
Fixed Maturities |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Investment income (loss) | 76.9 | 80.2 | 153.3 | 160.0
Equity Securities |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Investment income (loss) | 0.7 | 0.9 | 1.5 | 3.7
Total Alternative Investments |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Investment income (loss) | (2.2) | (8.5) | 0.8 | (7.2)
Equity Method Limited Liability Investments |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Investment income (loss) | (5.3) | (14.7) | (6.0) | (16.8)
Limited Liability Investments Included in Equity Securities |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Investment income (loss) | 3.1 | 6.2 | 6.8 | 9.6
Short-term Investments |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Investment income (loss) | 6.0 | 7.3 | 14.6 | 14.6
Loans to Policyholders |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Investment income (loss) | 5.1 | 5.1 | 10.4 | 10.3
Real Estate |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Investment income (loss) | 2.3 | 2.2 | 4.5 | 4.5
Investment expenses | 2.2 | 2.1 | 4.3 | 4.3
Company-Owned Life Insurance |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Investment income (loss) | 10.5 | 8.9 | 20.7 | 16.0
Universal Life-type Policyholder Account Balances |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Investment income (loss) | 3.3 | 2.6 | 5.3 | 5.1
Other Investment Expenses |  |  |  |
Net Investment Income [Line Items] |  |  |  |
Investment expenses | 4.5 | 3.6 | 9.7 | 9.3
Interest Expense, Other Short-Term Borrowings | $ 4.8 | $ 5.4 | $ 9.6 | $ 10.6

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