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Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions | Jun. 30, 2021 USD ($) | Dec. 31, 2020 USD ($)
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Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, available-for-sale | $ 6,555.0 | $ 6,345.3
Equity securities at fair value | 145.7 | 121.6
Municipal bonds |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, available-for-sale | 1,799.1 | 1,827.5
Corporate bonds |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, available-for-sale | 2,408.8 | 2,122.9
Other asset-backed securities |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, available-for-sale | 1,168.2 | 1,231.8
Government mortgage-backed securities |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, available-for-sale | 678.3 | $ 684.8
Level 3 |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Derivatives embedded in fixed indexed annuity products | $ 108.9 |
Level 3 | discounted cash flow | lapse rate | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.0530 |
Level 3 | discounted cash flow | mortality multiplier | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.6300 |
Level 3 | discounted cash flow | option budget | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.0090 |
Level 3 | discounted cash flow | option budget | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.0250 |
Level 3 | discounted cash flow | non-performance adjustment | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.0500 |
Level 3 | Black Scholes | equity value | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Equity securities, measurement input | 0.3100 |
Level 3 | Black Scholes | equity value | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Equity securities, measurement input | 0.4100 |
Level 3 | Municipal bonds | discounted cash flow | I Spread | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0307 |
Level 3 | Municipal bonds | discounted cash flow | I Spread | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0391 |
Level 3 | Corporate bonds | discounted cash flow | N Spread | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0272 |
Level 3 | Corporate bonds | discounted cash flow | N Spread | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0553 |
Level 3 | Corporate bonds | market comparable | option adjusted spread | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.1254 |
Level 3 | Other asset-backed securities | discounted cash flow | constant prepayment rate | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.2000 |
Level 3 | Other asset-backed securities | discounted cash flow | T spread | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0235 |
Level 3 | Other asset-backed securities | discounted cash flow | T spread | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0800 |
Level 3 | Other asset-backed securities | discounted cash flow | PDI interest margin | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0713 |
Level 3 | Other asset-backed securities | discounted cash flow | SBL interest margin | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0450 |
Level 3 | Other asset-backed securities | vendor price | haircut | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0300 |
Level 3 | Other asset-backed securities | vendor price | haircut | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0500 |
Level 3 | Government mortgage-backed securities | vendor price | haircut | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0300 |
Level 3 | Government mortgage-backed securities | vendor price | haircut | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Debt securities, measurement input | 0.0500 |

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