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Investments - Narrative (Details) - USD ($) $ in Millions | 6 Months Ended |  |
--- | --- | --- | ---
Jun. 30, 2024 | Jun. 30, 2023 | Dec. 31, 2023 | Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value | $ 1,632.2 |  |  |
Financing Receivable, Accrued Interest, before Allowance for Credit Loss | 75.4 |  | $ 77.0 |
Fixed maturities, allowance for credit losses | (7.5) | $ 7.1 | (8.2) | $ 9.6
Equity securities without readily determinable fair value, impairment loss, cumulative amount | $ 4.8 |  | 8.0 |
Percentage of Equity Method Limited Liability Investments reported with a lag | 3.30% |  |  |
Percentage of Equity Method Limited Liability Investments reported with a One Month Lag | 4.70% |  |  |
Loans to Policyholders | $ 279.8 |  | 281.2 |
Equity securities, FV-NI, unrealized gain (loss) | (0.6) | 9.8 |  |
Equity Securities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Other than Temporary Impairment Losses, Investments | 0.4 |  |  |
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 | 219.1 |  |  |
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 | 93.4 |  |  |
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 | 61.2 |  |  |
Other Non-Governmental Issuers |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt Securities, Available-for-Sale, Maturity, without Single Maturity Date, Fair Value | 1,258.5 |  |  |
Investments in Fixed Maturities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 880.4 |  | 730.3 |
12 Months or Longer | 851.0 |  | 721.2 |
Other Receivables | 3.6 |  | 0.9 |
Other liabilities | 18.9 |  | 0.0 |
Equity Securities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Other Receivables | 0.0 |  | 0.1 |
Corporate Bonds and Notes |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 509.9 |  | 383.8 |
12 Months or Longer | 487.0 |  | 378.3 |
Fixed maturities, allowance for credit losses | (7.2) | $ 6.7 | (7.7) | $ 8.9
Equity Method Investments |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
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 | 17.3 |  | 17.3 |
Consolidated Entity, Excluding VIE |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, allowance for credit losses | (7.5) |  | (8.2) |
Equity securities | 226.6 |  | 225.8 |
Other Receivables | 191.5 |  | 200.5 |
Equity Securities, FV-NI, Current | 226.6 |  | 225.8 |
Loans to Policyholders | 279.8 |  | 281.2 |
Consolidated Entity, Excluding VIE | Equity Method Limited Liability Investments |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Equity Method Investments | 205.1 |  | 221.7 |
Consolidated Entity, Excluding VIE | Alternative Energy Partnership Investments |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Equity Method Investments | 17.3 |  | 17.3 |
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 | 80.8 |  |  |
External Credit Rating, Investment Grade [Member] | Investments in Fixed Maturities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | 862.8 |  | 704.8 |
Non-investment-grade | Investments in Fixed Maturities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Debt securities, available-for-sale, unrealized loss position, accumulated loss | $ 17.6 |  | $ 25.5 |
Percentage of unrealized loss position to amortized cost basis of available for sale security average | 5.40% |  | 8.80% |

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