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Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions | Jun. 30, 2025 USD ($) | Mar. 31, 2025 USD ($) | Dec. 31, 2024 USD ($) | Jun. 30, 2024 USD ($) | Mar. 31, 2024 USD ($) | Dec. 31, 2023 USD ($)
--- | --- | --- | --- | --- | --- | ---
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | $ 5,453.3 |  | $ 5,387.9 |  |  |
Equity securities | 58.3 |  | 66.5 |  |  |
Limited partnership interests | 1,137.8 |  | 1,121.3 |  |  |
Net MRBs | (6.7) | $ (5.2) | (6.8) | $ (6.2) | $ (6.3) | $ (3.9)
Mortgage-backed securities |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | 605.5 |  | 755.8 |  |  |
Municipal bonds |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | 1,151.9 |  | 1,150.8 |  |  |
Corporate bonds |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | 1,770.8 |  | 1,782.4 |  |  |
Other asset-backed securities |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | $ 1,548.2 |  | 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 | 0.034 |  |  |  |  |
Level 3 | Corporate bonds | discounted cash flow | yield | Maximum |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Corporate bonds, measurement input | 0.092 |  |  |  |  |
Level 3 | Corporate bonds | discounted cash flow | option adjusted spread | Minimum |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Corporate bonds, measurement input | 0.0282 |  |  |  |  |
Level 3 | Corporate bonds | discounted cash flow | option adjusted spread | Maximum |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Corporate bonds, measurement input | 0.0808 |  |  |  |  |
Recurring |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Limited partnership interests | $ 36.6 |  | 28.9 |  |  |
Recurring | Fair Value |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | 5,453.3 |  | 5,387.9 |  |  |
Equity securities | 58.3 |  | 66.5 |  |  |
Limited partnership interests | 33.7 |  | 28.9 |  |  |
Financial Liabilities | 78.1 |  | 79.6 |  |  |
Recurring | Fair Value | Mortgage-backed securities |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | 605.5 |  | 755.8 |  |  |
Recurring | Fair Value | Municipal bonds |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | 1,151.9 |  | 1,150.8 |  |  |
Recurring | Fair Value | Corporate bonds |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | 1,770.8 |  | 1,782.4 |  |  |
Recurring | Fair Value | Other asset-backed securities |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | 1,548.2 |  | 1,328.2 |  |  |
Recurring | Fair Value | Level 3 |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | 481.1 |  | 499.9 |  |  |
Equity securities | 4.3 |  | 4.2 |  |  |
Limited partnership interests | 33.7 |  | 28.9 |  |  |
Totals | 519.1 |  | 533.0 |  |  |
Financial Liabilities | 74.0 |  | 75.5 |  |  |
Derivatives embedded in fixed indexed annuity products | 80.7 |  |  |  |  |
Net MRBs | (6.7) |  |  |  |  |
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 | 75.9 |  | 74.9 |  |  |
Recurring | Fair Value | Level 3 | Corporate bonds |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | 342.0 |  | 351.3 |  |  |
Recurring | Fair Value | Level 3 | Other asset-backed securities |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | 63.2 |  | 73.7 |  |  |
Recurring | Fair Value | Level 3 | Internal |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | 203.7 |  | 199.3 |  |  |
Equity securities | 0.0 |  | 0.0 |  |  |
Limited partnership interests | 0.0 |  | 0.0 |  |  |
Totals | 203.7 |  | 199.3 |  |  |
Financial Liabilities | 74.0 |  | 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 | 203.7 |  | 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 | 277.4 |  | 300.6 |  |  |
Equity securities | 4.3 |  | 4.2 |  |  |
Limited partnership interests | 33.7 |  | 28.9 |  |  |
Totals | 315.4 |  | 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 | 75.9 |  | 74.9 |  |  |
Recurring | Fair Value | Level 3 | External | Corporate bonds |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | 138.3 |  | 152.0 |  |  |
Recurring | Fair Value | Level 3 | External | Other asset-backed securities |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Fair Value | $ 63.2 |  | $ 73.7 |  |  |

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- Definition Fair value of financial obligations, including, but not limited to, debt instruments, derivative liabilities, federal funds purchased and sold under agreements to repurchase, securities loaned or sold under agreements to repurchase, financial instruments sold not yet purchased, guarantees, line of credit, loans and notes payable, servicing liability, and trading liabilities. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 28 -Subparagraph (c)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-28 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 11 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-11 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 11 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-11 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 10 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-10 Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2E -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2E + Details
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- Definition Amount, before effect of reinsurance and after accumulated increase (decrease) in instrument-specific credit risk, of contract or contract feature in long-duration contract issued by insurance entity that both protects contract holder from other-than-nominal capital market risk and exposes insurance entity to other-than-nominal capital market risk. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (h)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480016/944-40-65-2 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 29G -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-29G Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 7B -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-7B Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 45 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480112/944-40-45-3 + Details
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Data Type: | na
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