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Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions | Mar. 31, 2024 USD ($) | Dec. 31, 2023 USD ($) | Mar. 31, 2023 USD ($) | Dec. 31, 2022 USD ($)
--- | --- | --- | --- | ---
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | $ 5,342.7 | $ 5,235.3 |  |
Equity securities at fair value, (cost $101700000 and $86200000) | 84.7 | 86.2 |  |
Net MRBs | (6.3) | (3.9) | $ 1.3 | $ 0.2
Fair Value | Recurring |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 5,342.7 | 5,235.3 |  |
Equity securities at fair value, (cost $101700000 and $86200000) | 84.7 | 86.2 |  |
Financial Liabilities | 85.0 | 86.0 |  |
Mortgage-backed securities |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 677.7 | 653.2 |  |
Mortgage-backed securities | Fair Value | Recurring |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 677.7 | 653.2 |  |
Municipal bonds |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 1,207.5 | 1,270.1 |  |
Municipal bonds | Fair Value | Recurring |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 1,207.5 | 1,270.1 |  |
Corporate bonds |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 1,829.9 | 1,772.7 |  |
Corporate bonds | Fair Value | Recurring |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 1,829.9 | 1,772.7 |  |
Other asset-backed securities |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 1,214.5 | 1,128.4 |  |
Other asset-backed securities | Fair Value | Recurring |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 1,214.5 | 1,128.4 |  |
Level 3 | Fair Value | Recurring |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 507.4 | 514.0 |  |
Equity securities at fair value, (cost $101700000 and $86200000) | 2.5 | 4.5 |  |
Totals | 509.9 | 518.5 |  |
Financial Liabilities | 80.8 | 82.4 |  |
Derivatives embedded in fixed indexed annuity products | 87.1 |  |  |
Net MRBs | (6.3) |  |  |
Level 3 | Fair Value | Recurring | Internal |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 185.5 | 180.4 |  |
Equity securities at fair value, (cost $101700000 and $86200000) | 0.0 | 0.0 |  |
Totals | 185.5 | 180.4 |  |
Financial Liabilities | 80.8 | 82.4 |  |
Level 3 | Fair Value | Recurring | External |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 321.9 | 333.6 |  |
Equity securities at fair value, (cost $101700000 and $86200000) | 2.5 | 4.5 |  |
Totals | 324.4 | 338.1 |  |
Financial Liabilities | 0.0 | 0.0 |  |
Level 3 | Mortgage-backed securities | Fair Value | Recurring |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 0.0 | 0.0 |  |
Level 3 | Mortgage-backed securities | Fair Value | Recurring | Internal |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 0.0 | 0.0 |  |
Level 3 | Mortgage-backed securities | Fair Value | Recurring | External |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 0.0 | 0.0 |  |
Level 3 | Municipal bonds | Fair Value | Recurring |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 73.2 | 74.0 |  |
Level 3 | Municipal bonds | Fair Value | Recurring | Internal |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 0.0 | 0.0 |  |
Level 3 | Municipal bonds | Fair Value | Recurring | External |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 73.2 | 74.0 |  |
Level 3 | Corporate bonds | Fair Value | Recurring |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 345.5 | 342.5 |  |
Level 3 | Corporate bonds | Fair Value | Recurring | Internal |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 185.5 | 180.4 |  |
Level 3 | Corporate bonds | Fair Value | Recurring | External |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 160.0 | 162.1 |  |
Level 3 | Other asset-backed securities | Fair Value | Recurring |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 88.7 | 97.5 |  |
Level 3 | Other asset-backed securities | Fair Value | Recurring | Internal |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | 0.0 | 0.0 |  |
Level 3 | Other asset-backed securities | Fair Value | Recurring | External |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Fair Value | $ 88.7 | $ 97.5 |  |
Minimum | Level 3 | discounted cash flow | option budget |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.009 |  |  |
Minimum | Level 3 | Corporate bonds | discounted cash flow | yield |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Debt securities, measurement input | 0.045 |  |  |
Minimum | Level 3 | Corporate bonds | discounted cash flow | option adjusted spread |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Debt securities, measurement input | 0.0116 |  |  |
Maximum | Level 3 | discounted cash flow | option budget |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.038 |  |  |
Maximum | Level 3 | Corporate bonds | discounted cash flow | yield |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Debt securities, measurement input | 0.162 |  |  |
Maximum | Level 3 | Corporate bonds | discounted cash flow | option adjusted spread |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Debt securities, measurement input | 0.0852 |  |  |
Weighted Average | Level 3 | discounted cash flow | lapse rate |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.059 |  |  |
Net MRBs, measurement input | 0.059 |  |  |
Weighted Average | Level 3 | discounted cash flow | mortality multiplier |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.694 |  |  |
Net MRBs, measurement input | 0.694 |  |  |
Weighted Average | Level 3 | discounted cash flow | non-performance adjustment |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.050 |  |  |

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