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Fair Value of Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Details) $ in Millions | Dec. 31, 2024 USD ($) | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($)
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Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | $ 5,387.9 | $ 5,235.3 |
Equity securities at fair value, (cost $78.8 and $86.2) | 66.5 | 86.2 |
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) | 1,121.3 | 1,138.8 |
Net MRBs | (6.8) | (3.9) | $ 0.3
Recurring |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) | 28.9 | 29.5 |
Fair Value | Recurring |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 5,387.9 | 5,235.3 |
Equity securities at fair value, (cost $78.8 and $86.2) | 66.5 | 86.2 |
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) | 28.9 |  |
Financial Liabilities | 79.6 | 86.0 |
Mortgage-backed securities | Fair Value | Recurring |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 755.8 | 653.2 |
Municipal bonds | Fair Value | Recurring |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 1,150.8 | 1,270.1 |
Corporate bonds | Fair Value | Recurring |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 1,782.4 | 1,772.7 |
Other asset-backed securities | Fair Value | Recurring |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 1,328.2 | 1,128.4 |
Fair Value, Inputs, Level 3 | Fair Value | Recurring |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 499.9 | 514.0 |
Equity securities at fair value, (cost $78.8 and $86.2) | 4.2 | 4.5 |
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) | 28.9 |  |
Totals | 533.0 | 518.5 |
Financial Liabilities | 75.5 | 82.4 |
Derivatives embedded in fixed indexed annuity products | 82.3 | 86.3 |
Net MRBs | (6.8) | (3.9) |
Fair Value, Inputs, Level 3 | Fair Value | Recurring | Internal |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 199.3 | 180.4 |
Equity securities at fair value, (cost $78.8 and $86.2) | 0.0 | 0.0 |
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) | 0.0 |  |
Totals | 199.3 | 180.4 |
Financial Liabilities | 75.5 | 82.4 |
Fair Value, Inputs, Level 3 | Fair Value | Recurring | External |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 300.6 | 333.6 |
Equity securities at fair value, (cost $78.8 and $86.2) | 4.2 | 4.5 |
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) | 28.9 |  |
Totals | 333.7 | 338.1 |
Financial Liabilities | 0.0 | 0.0 |
Fair Value, Inputs, Level 3 | Mortgage-backed securities | Fair Value | Recurring |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 0.0 | 0.0 |
Fair Value, Inputs, Level 3 | Mortgage-backed securities | Fair Value | Recurring | Internal |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 0.0 | 0.0 |
Fair Value, Inputs, Level 3 | Mortgage-backed securities | Fair Value | Recurring | External |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 0.0 | 0.0 |
Fair Value, Inputs, Level 3 | Municipal bonds | Fair Value | Recurring |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 74.9 | 74.0 |
Fair Value, Inputs, Level 3 | Municipal bonds | Fair Value | Recurring | Internal |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 0.0 | 0.0 |
Fair Value, Inputs, Level 3 | Municipal bonds | Fair Value | Recurring | External |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 74.9 | 74.0 |
Fair Value, Inputs, Level 3 | Corporate bonds | Fair Value | Recurring |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 351.3 | 342.5 |
Fair Value, Inputs, Level 3 | Corporate bonds | Fair Value | Recurring | Internal |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 199.3 | 180.4 |
Fair Value, Inputs, Level 3 | Corporate bonds | Fair Value | Recurring | External |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 152.0 | 162.1 |
Fair Value, Inputs, Level 3 | Other asset-backed securities | Fair Value | Recurring |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 73.7 | 97.5 |
Fair Value, Inputs, Level 3 | Other asset-backed securities | Fair Value | Recurring | Internal |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | 0.0 | 0.0 |
Fair Value, Inputs, Level 3 | Other asset-backed securities | Fair Value | Recurring | External |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Fixed maturity securities, available for sale, at fair value (amortized cost, net 2024, $5,842.5; 2023, $5,652.9) | $ 73.7 | $ 97.5 |
Minimum | Fair Value, Inputs, Level 3 | discounted cash flow | Measurement Input Option Budget |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.009 | 0.009 |
Minimum | Fair Value, Inputs, Level 3 | Corporate bonds | discounted cash flow | yield |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Debt securities, measurement input | 0.039 | 0.067 |
Minimum | Fair Value, Inputs, Level 3 | Corporate bonds | discounted cash flow | option adjusted spread |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Debt securities, measurement input | 0.0242 |  |
Maximum | Fair Value, Inputs, Level 3 | discounted cash flow | Measurement Input Option Budget |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.038 | 0.038 |
Maximum | Fair Value, Inputs, Level 3 | Corporate bonds | discounted cash flow | yield |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Debt securities, measurement input | 0.106 | 0.170 |
Maximum | Fair Value, Inputs, Level 3 | Corporate bonds | discounted cash flow | option adjusted spread |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Debt securities, measurement input | 0.0768 |  |
Weighted Average | Fair Value, Inputs, Level 3 | discounted cash flow | lapse rate |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.064 | 0.059 |
Net MRBs, measurement input | 0.064 | 0.059 |
Weighted Average | Fair Value, Inputs, Level 3 | discounted cash flow | mortality multiplier |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |
Derivatives embedded in fixed indexed annuity products, measurement input | 0.670 | 0.694 |
Net MRBs, measurement input | 0.670 | 0.678 |
Weighted Average | Fair Value, Inputs, 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 | 0.050 |

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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 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2E -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2E Reference 2: 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 3: 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 + Details
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