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Investments - Fair Value and Gross Unrealized Losses (Details) $ in Millions | Mar. 31, 2024 USD ($) | Mar. 31, 2024 security | Mar. 31, 2024 | Mar. 31, 2024 position | Dec. 31, 2023 USD ($) | Dec. 31, 2023 security | Dec. 31, 2023 | Dec. 31, 2023 position
--- | --- | --- | --- | --- | --- | --- | --- | ---
Fixed maturity securities, Gross Unrealized Losses |  |  |  |  |  |  |  |
Number of positions with a gross unrealized loss, 12 Months or Less (security) | security |  | 298 |  |  |  | 195 |  |
Number of positions with a gross unrealized loss, more than 12 months (security) |  | 2,213 |  | 2,213 |  | 2,305 |  | 2,305
Number of positions with a gross unrealized loss, Total (security) | security |  | 2,511 |  |  |  | 2,500 |  |
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 Months or Less (percent) |  |  | 10.90% |  |  |  | 9.20% |
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months (percent) |  |  | 59.10% |  |  |  | 65.20% |
Fair value as a percentage of total fixed maturities and equity securities fair value, Total (percent) |  |  | 70.00% |  |  |  | 74.40% |
Fixed maturity securities |  |  |  |  |  |  |  |
Fixed maturity securities, Fair Value |  |  |  |  |  |  |  |
Fixed maturity securities, Fair Value, 12 Months or Less | $ 581.8 |  |  |  | $ 479.2 |  |  |
Fixed maturity securities, Fair Value, More than 12 Months | 3,155.2 |  |  |  | 3,415.5 |  |  |
Fixed maturity securities, Fair Value, Total | 3,737.0 |  |  |  | 3,894.7 |  |  |
Fixed maturity securities, Gross Unrealized Losses |  |  |  |  |  |  |  |
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less | 28.4 |  |  |  | 28.3 |  |  |
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months | 471.0 |  |  |  | 452.2 |  |  |
Fixed maturity securities, Gross Unrealized Losses, Total | 499.4 |  |  |  | 480.5 |  |  |
Mortgage-backed securities |  |  |  |  |  |  |  |
Fixed maturity securities, Fair Value |  |  |  |  |  |  |  |
Fixed maturity securities, Fair Value, 12 Months or Less | 63.1 |  |  |  | 45.3 |  |  |
Fixed maturity securities, Fair Value, More than 12 Months | 470.4 |  |  |  | 458.5 |  |  |
Fixed maturity securities, Fair Value, Total | 533.5 |  |  |  | 503.8 |  |  |
Fixed maturity securities, Gross Unrealized Losses |  |  |  |  |  |  |  |
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less | 1.0 |  |  |  | 0.8 |  |  |
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months | 70.7 |  |  |  | 63.8 |  |  |
Fixed maturity securities, Gross Unrealized Losses, Total | 71.7 |  |  |  | 64.6 |  |  |
Other |  |  |  |  |  |  |  |
Fixed maturity securities, Fair Value |  |  |  |  |  |  |  |
Fixed maturity securities, Fair Value, 12 Months or Less | 100.8 |  |  |  | 39.5 |  |  |
Fixed maturity securities, Fair Value, More than 12 Months | 276.6 |  |  |  | 288.0 |  |  |
Fixed maturity securities, Fair Value, Total | 377.4 |  |  |  | 327.5 |  |  |
Fixed maturity securities, Gross Unrealized Losses |  |  |  |  |  |  |  |
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less | 1.6 |  |  |  | 0.4 |  |  |
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months | 65.2 |  |  |  | 62.4 |  |  |
Fixed maturity securities, Gross Unrealized Losses, Total | 66.8 |  |  |  | 62.8 |  |  |
Municipal bonds |  |  |  |  |  |  |  |
Fixed maturity securities, Fair Value |  |  |  |  |  |  |  |
Fixed maturity securities, Fair Value, 12 Months or Less | 111.7 |  |  |  | 64.5 |  |  |
Fixed maturity securities, Fair Value, More than 12 Months | 704.9 |  |  |  | 724.6 |  |  |
Fixed maturity securities, Fair Value, Total | 816.6 |  |  |  | 789.1 |  |  |
Fixed maturity securities, Gross Unrealized Losses |  |  |  |  |  |  |  |
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less | 1.6 |  |  |  | 0.9 |  |  |
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months | 95.7 |  |  |  | 91.0 |  |  |
Fixed maturity securities, Gross Unrealized Losses, Total | 97.3 |  |  |  | 91.9 |  |  |
Foreign government bonds |  |  |  |  |  |  |  |
Fixed maturity securities, Fair Value |  |  |  |  |  |  |  |
Fixed maturity securities, Fair Value, 12 Months or Less | 1.4 |  |  |  | 1.5 |  |  |
Fixed maturity securities, Fair Value, More than 12 Months | 20.5 |  |  |  | 20.6 |  |  |
Fixed maturity securities, Fair Value, Total | 21.9 |  |  |  | 22.1 |  |  |
Fixed maturity securities, Gross Unrealized Losses |  |  |  |  |  |  |  |
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less | 0.1 |  |  |  | 0.0 |  |  |
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months | 1.1 |  |  |  | 1.0 |  |  |
Fixed maturity securities, Gross Unrealized Losses, Total | 1.2 |  |  |  | 1.0 |  |  |
Corporate bonds |  |  |  |  |  |  |  |
Fixed maturity securities, Fair Value |  |  |  |  |  |  |  |
Fixed maturity securities, Fair Value, 12 Months or Less | 252.8 |  |  |  | 195.0 |  |  |
Fixed maturity securities, Fair Value, More than 12 Months | 1,147.4 |  |  |  | 1,171.3 |  |  |
Fixed maturity securities, Fair Value, Total | 1,400.2 |  |  |  | 1,366.3 |  |  |
Fixed maturity securities, Gross Unrealized Losses |  |  |  |  |  |  |  |
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less | 23.5 |  |  |  | 25.4 |  |  |
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months | 206.2 |  |  |  | 194.9 |  |  |
Fixed maturity securities, Gross Unrealized Losses, Total | 229.7 |  |  |  | 220.3 |  |  |
Other asset-backed securities |  |  |  |  |  |  |  |
Fixed maturity securities, Fair Value |  |  |  |  |  |  |  |
Fixed maturity securities, Fair Value, 12 Months or Less | 52.0 |  |  |  | 133.4 |  |  |
Fixed maturity securities, Fair Value, More than 12 Months | 535.4 |  |  |  | 752.5 |  |  |
Fixed maturity securities, Fair Value, Total | 587.4 |  |  |  | 885.9 |  |  |
Fixed maturity securities, Gross Unrealized Losses |  |  |  |  |  |  |  |
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less | 0.6 |  |  |  | 0.8 |  |  |
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months | 32.1 |  |  |  | 39.1 |  |  |
Fixed maturity securities, Gross Unrealized Losses, Total | $ 32.7 |  |  |  | $ 39.9 |  |  |

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