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Investments - Narrative (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2024 USD ($) | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($)
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Other receivables | $ 185.7 | $ 200.5 |
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date | $ 1,537.3 |  |
Debt Securities, Available-for-Sale, Unrealized Loss Amount, Average, Percentage | 0.049 |  |
Financing Receivable, Accrued Interest, before Allowance for Credit Loss | $ 70.9 | 77.0 |
Equity Securities, FV-NI, Current | 218.5 | 225.8 |
Equity securities, FV-NI, unrealized gain (loss) | (5.1) | 3.0 |
Convertible Securities at Fair Value | 280.7 | 281.2 |
Debt and equity securities, gain (loss), excluding other-than-temporary impairment loss | 3.2 | 8.0 |
Outstanding commitments to fund equity method limited liability investments | $ 91.3 |  |
Equity method investments, reported without a reporting lag | 0.031 |  |
Equity method investments, reported with a one-moth reporting lag | 0.041 |  |
Equity Securities at Modified Cost | $ 539.2 | 513.5 |
Other Asset Impairment Charges | (5.8) | (1.1) | $ (25.8)
Other Investments and Securities, at Cost | 22.5 | 32.6 |
Alternative Energy Partnership Investments |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Equity Method Investments | 17.6 | 17.3 |
Equity Method Limited Liability Investments |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Equity Method Investments | 186.3 | 221.7 |
Dividends on Equity Securities Excluding Alternative Investments |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Other Asset Impairment Charges | (0.4) | (0.5) | 0.0
Other than Temporary Impairment Losses, Investments |  |  | $ 0.0
Investments in Fixed Maturities |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Other receivables | 1.8 | 0.9 |
Other liabilities | 11.6 | 0.0 |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 889.5 | 730.3 |
Government National Mortgage Association Certificates and Obligations (GNMA) |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date | 207.0 |  |
Federal National Mortgage Association Certificates and Obligations (FNMA) |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date | 99.5 |  |
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date | 76.3 |  |
Other Non-Governmental Issuers |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date | 1,154.5 |  |
Foreign Governments |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 0.2 | 0.6 |
Bonds and Notes |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 518.8 | 383.8 |
Equity Method Limited Liability Investments at Cost Plus Cumulative Undistributed Earnings |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Other receivables | 0.0 | 0.0 |
Other liabilities | 0.0 | 0.0 |
Dividends on Equity Securities Excluding Alternative Investments |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Other receivables | 0.3 | 0.1 |
Other liabilities | 0.0 | 0.0 |
Investment-grade | Investments in Fixed Maturities |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 875.3 | 704.8 |
Non-investment-grade | Investments in Fixed Maturities |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | $ 14.2 | $ 25.5 |
Percentage of unrealized loss position to amortized cost basis of available for sale security average |  | 8.80% |

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- Definition This item represents the amount that could be realized under a life insurance contract or contracts owned by the Entity as of the date of the statement of financial position. Such Entity-owned life insurance policies are commonly known as corporate-owned life insurance (COLI) or bank-owned life insurance (BOLI). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(12)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 325 -SubTopic 30 -Name Accounting Standards Codification -Section 25 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481612/325-30-25-1 + Details
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- Definition Amount before allowance of loans made to policy and contract holders using their life insurance contracts' cash value or separate accounts as collateral. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums over and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)(e)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)(6)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1 + Details
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