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Investments - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022 | Dec. 31, 2022 | Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value | $ 1,637.0 |  | $ 1,637.0 |  |  |
Fixed maturities, allowance for credit losses | (8.6) | $ 7.5 | (8.6) | $ 7.5 | $ (9.6) | $ 7.5
Equity securities | 233.4 |  | 233.4 |  | 243.2 |
Equity securities without readily determinable fair value, impairment loss, cumulative amount | 7.5 |  | 7.5 |  |  |
Other Receivables | 223.4 |  | 223.4 |  | 262.6 |
Equity Securities, FV-NI, Current | $ 233.4 |  | $ 233.4 |  | 243.2 |
Percentage of Equity Method Limited Liability Investments reported with a lag | 2.50% |  | 2.50% |  |  |
Percentage of Equity Method Limited Liability Investments reported with a One Month Lag | 5.50% |  | 5.50% |  |  |
Company-Owned Life Insurance | $ 506.9 |  | $ 506.9 |  | 586.5 |
Loans to Policyholders | 281.8 |  | 281.8 |  |  |
Loans to Policyholders | 281.8 |  | 281.8 |  | 283.4 |
Government National Mortgage Association Certificates and Obligations (GNMA) |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value | 231.3 |  | 231.3 |  |  |
Federal National Mortgage Association Certificates and Obligations (FNMA) |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value | 85.9 |  | 85.9 |  |  |
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value | 59.2 |  | 59.2 |  |  |
Other Non-Governmental Issuers |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value | 1,260.6 |  | 1,260.6 |  |  |
Investments in Fixed Maturities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 1,149.0 |  | 1,149.0 |  | 936.8 |
12 Months or Longer | 1,068.1 |  | 1,068.1 |  | 308.9 |
Other Receivables | 0.9 |  | 0.9 |  | 5.8 |
Other liabilities | 1.0 |  | 1.0 |  | 25.9 |
Equity Securities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Other Receivables | 20.7 |  | 20.7 |  | 0.0 |
Equity Securities | Fair Value, Measurements, Recurring |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Equity securities | 233.4 |  | 233.4 |  | 243.2 |
Equity Securities, FV-NI, Current | 233.4 |  | 233.4 |  | 243.2 |
Equity Securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Equity securities | 7.9 |  | 7.9 |  | 13.4 |
Equity Securities, FV-NI, Current | 7.9 |  | 7.9 |  | 13.4 |
Preferred Stocks | Other Industries | Fair Value, Measurements, Recurring |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Equity securities | 11.7 |  | 11.7 |  | 10.8 |
Equity Securities, FV-NI, Current | 11.7 |  | 11.7 |  | 10.8 |
Preferred Stocks | Other Industries | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Equity securities | 0.0 |  | 0.0 |  | 0.0 |
Equity Securities, FV-NI, Current | 0.0 |  | 0.0 |  | 0.0 |
Common Stock | Other Industries | Fair Value, Measurements, Recurring |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Equity securities | 0.6 |  | 0.6 |  | 1.2 |
Equity Securities, FV-NI, Current | 0.6 |  | 0.6 |  | 1.2 |
Common Stock | Other Industries | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Equity securities | 0.2 |  | 0.2 |  | 0.3 |
Equity Securities, FV-NI, Current | 0.2 |  | 0.2 |  | 0.3 |
Corporate Debt Securities [Member] |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 651.6 |  | 651.6 |  | 499.7 |
12 Months or Longer | 597.5 |  | 597.5 |  | 125.8 |
Fixed maturities, allowance for credit losses | (8.1) | 7.1 | (8.1) | 7.1 | (8.9) | $ 7.5
Equity Method Investments |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Other Receivables | 2.1 |  | 2.1 |  | 35.2 |
Other liabilities | 0.0 |  | 0.0 |  |  |
Variable Interest Entity, Not Primary Beneficiary |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount | 17.2 |  | 17.2 |  | 16.3 |
Variable Interest Entity, Funding Investment Commitment, Funding | 0.0 | $ 0.0 | 0.0 | $ 0.0 |  |
Variable Interest Entity, Not Primary Beneficiary | Equity Method Limited Liability Investments |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Outstanding commitments to fund equity method limited liability investments | 89.4 |  | 89.4 |  |  |
Variable Interest Entity, Not Primary Beneficiary | Alternative Energy Partnership Investments |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Outstanding commitments to fund equity method limited liability investments | 0.0 |  | 0.0 |  | 0.0 |
Investment-grade | Investments in Fixed Maturities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 1,120.1 |  | 1,120.1 |  | 904.0 |
Non-investment-grade | Investments in Fixed Maturities |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | $ 28.9 |  | $ 28.9 |  | $ 32.8 |
Percentage of unrealized loss position to amortized cost basis of available for sale security average | 9.40% |  | 9.40% |  | 11.00% |

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