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Investments - Schedule of Fixed Maturities (Details) - USD ($) $ in Millions | Jun. 30, 2024 | Dec. 31, 2023 | Jun. 30, 2023 | Dec. 31, 2022
--- | --- | --- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | $ 7,546.9 |  |  |
Gross unrealized gains | 15.7 |  |  |
Gross unrealized loss | (880.4) |  |  |
Fixed maturities, allowance for credit losses | (7.5) | $ (8.2) | $ 7.1 | $ 9.6
Fixed maturities at fair value | 6,674.7 |  |  |
Investments in Fixed Maturities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost |  | 7,565.8 |  |
Gross unrealized gains |  | 54.6 |  |
Gross unrealized loss |  | (730.3) |  |
Fixed maturities at fair value |  | 6,881.9 |  |
U.S. Government and Government Agencies and Authorities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 574.9 | 594.1 |  |
Gross unrealized gains | 0.7 | 1.9 |  |
Gross unrealized loss | (100.1) | (84.5) |  |
Fixed maturities, allowance for credit losses | 0.0 | 0.0 |  |
Fixed maturities at fair value | 475.5 | 511.5 |  |
States and Political Subdivisions |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 1,550.9 | 1,575.9 |  |
Gross unrealized gains | 4.5 | 16.3 |  |
Gross unrealized loss | (215.4) | (189.8) |  |
Fixed maturities, allowance for credit losses | (0.3) | (0.5) | 0.4 | 0.7
Fixed maturities at fair value | 1,339.7 | 1,401.9 |  |
Foreign Governments |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 5.1 | 4.4 |  |
Gross unrealized gains | 0.0 | 0.0 |  |
Gross unrealized loss | (0.5) | (0.6) |  |
Fixed maturities, allowance for credit losses | 0.0 | 0.0 |  |
Fixed maturities at fair value | 4.6 | 3.8 |  |
Corporate Bonds and Notes |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 4,095.4 | 4,046.8 |  |
Gross unrealized gains | 8.3 | 35.5 |  |
Gross unrealized loss | (509.9) | (383.8) |  |
Fixed maturities, allowance for credit losses | (7.2) | (7.7) | $ 6.7 | $ 8.9
Fixed maturities at fair value | 3,586.6 | 3,690.8 |  |
Redeemable Preferred Stocks |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 11.0 | 9.0 |  |
Gross unrealized gains | 0.1 | 0.1 |  |
Gross unrealized loss | (1.2) | (0.8) |  |
Fixed maturities, allowance for credit losses | 0.0 | 0.0 |  |
Fixed maturities at fair value | 9.9 | 8.3 |  |
Collateralized Loan Obligations |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 896.4 | 973.6 |  |
Gross unrealized gains | 2.0 | 0.7 |  |
Gross unrealized loss | (12.5) | (24.5) |  |
Fixed maturities, allowance for credit losses | 0.0 | 0.0 |  |
Fixed maturities at fair value | 885.9 | 949.8 |  |
Other Mortgage- and Asset-backed |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Fixed maturities, at amortized cost | 413.2 | 362.0 |  |
Gross unrealized gains | 0.1 | 0.1 |  |
Gross unrealized loss | (40.8) | (46.3) |  |
Fixed maturities, allowance for credit losses | 0.0 | 0.0 |  |
Fixed maturities at fair value | $ 372.5 | $ 315.8 |  |

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