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Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions | Mar. 31, 2025 USD ($) | Dec. 31, 2024 USD ($) | Mar. 31, 2024 USD ($) | Dec. 31, 2023 USD ($)
--- | --- | --- | --- | ---
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | $ 5,457.7 | $ 5,387.9 |  |
Equity securities | 64.3 | 66.5 |  |
Limited partnership interests (Carried under Fair Value Option, 2025 $34900000 and 2024, $28900000) | 1,125.6 | 1,121.3 |  |
Net MRBs | (5.2) | (6.8) | $ (6.3) | $ (3.9)
Mortgage-backed securities |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 812.2 | 755.8 |  |
Municipal bonds |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 1,155.9 | 1,150.8 |  |
Corporate bonds |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 1,779.9 | 1,782.4 |  |
Other asset-backed securities |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | $ 1,333.1 | 1,328.2 |  |
Level 3 | discounted cash flow | lapse rate | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.064 |  |  |
Net MRBs, measurement input | 0.064 |  |  |
Level 3 | discounted cash flow | mortality multiplier | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.670 |  |  |
Net MRBs, measurement input | 0.670 |  |  |
Level 3 | discounted cash flow | option budget | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.009 |  |  |
Level 3 | discounted cash flow | option budget | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.038 |  |  |
Level 3 | discounted cash flow | non-performance adjustment | Weighted Average |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.050 |  |  |
Level 3 | Corporate bonds | discounted cash flow | yield | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Corporate bonds, measurement input | 0.039 |  |  |
Level 3 | Corporate bonds | discounted cash flow | yield | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Corporate bonds, measurement input | 0.090 |  |  |
Level 3 | Corporate bonds | discounted cash flow | option adjusted spread | Minimum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Corporate bonds, measurement input | 0.0283 |  |  |
Level 3 | Corporate bonds | discounted cash flow | option adjusted spread | Maximum |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Corporate bonds, measurement input | 0.0809 |  |  |
Recurring |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Limited partnership interests (Carried under Fair Value Option, 2025 $34900000 and 2024, $28900000) | $ 34.9 | 28.9 |  |
Recurring | Fair Value |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 5,457.7 | 5,387.9 |  |
Equity securities | 64.3 | 66.5 |  |
Limited partnership interests (Carried under Fair Value Option, 2025 $34900000 and 2024, $28900000) | 34.9 | 28.9 |  |
Financial Liabilities | 76.9 | 79.6 |  |
Recurring | Fair Value | Mortgage-backed securities |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 812.2 | 755.8 |  |
Recurring | Fair Value | Municipal bonds |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 1,155.9 | 1,150.8 |  |
Recurring | Fair Value | Corporate bonds |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 1,780.0 | 1,782.4 |  |
Recurring | Fair Value | Other asset-backed securities |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 1,333.0 | 1,328.2 |  |
Recurring | Fair Value | Level 3 |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 484.1 | 499.9 |  |
Equity securities | 4.0 | 4.2 |  |
Limited partnership interests (Carried under Fair Value Option, 2025 $34900000 and 2024, $28900000) | 34.9 | 28.9 |  |
Totals | 523.0 | 533.0 |  |
Financial Liabilities | 74.3 | 75.5 |  |
Derivatives embedded in fixed indexed annuity products | 79.5 |  |  |
Net MRBs | (5.2) |  |  |
Recurring | Fair Value | Level 3 | Mortgage-backed securities |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 0.0 | 0.0 |  |
Recurring | Fair Value | Level 3 | Municipal bonds |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 76.1 | 74.9 |  |
Recurring | Fair Value | Level 3 | Corporate bonds |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 340.9 | 351.3 |  |
Recurring | Fair Value | Level 3 | Other asset-backed securities |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 67.1 | 73.7 |  |
Recurring | Fair Value | Level 3 | Internal |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 190.0 | 199.3 |  |
Equity securities | 0.0 | 0.0 |  |
Limited partnership interests (Carried under Fair Value Option, 2025 $34900000 and 2024, $28900000) | 0.0 |  |  |
Totals | 190.0 | 199.3 |  |
Financial Liabilities | 74.3 | 75.5 |  |
Recurring | Fair Value | Level 3 | Internal | Mortgage-backed securities |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 0.0 | 0.0 |  |
Recurring | Fair Value | Level 3 | Internal | Municipal bonds |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 0.0 | 0.0 |  |
Recurring | Fair Value | Level 3 | Internal | Corporate bonds |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 190.0 | 199.3 |  |
Recurring | Fair Value | Level 3 | Internal | Other asset-backed securities |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 0.0 | 0.0 |  |
Recurring | Fair Value | Level 3 | External |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 294.1 | 300.6 |  |
Equity securities | 4.0 | 4.2 |  |
Limited partnership interests (Carried under Fair Value Option, 2025 $34900000 and 2024, $28900000) | 34.9 | 28.9 |  |
Totals | 333.0 | 333.7 |  |
Financial Liabilities | 0.0 | 0.0 |  |
Recurring | Fair Value | Level 3 | External | Mortgage-backed securities |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 0.0 | 0.0 |  |
Recurring | Fair Value | Level 3 | External | Municipal bonds |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 76.1 | 74.9 |  |
Recurring | Fair Value | Level 3 | External | Corporate bonds |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 150.9 | 152.0 |  |
Recurring | Fair Value | Level 3 | External | Other asset-backed securities |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | $ 67.1 | $ 73.7 |  |

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