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Investments - Other Investments (Details) - USD ($) $ in Millions | 1 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Mar. 31, 2021 | Jun. 30, 2024 | Sep. 30, 2024 | Sep. 30, 2023 | Dec. 31, 2023 | Dec. 31, 2022
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |
Opening undistributed value of investment, beginning balance |  | $ 7.6 | $ 7.6 | $ 6.2 | $ 6.2 | $ 3.9
Investment in the period |  |  | 0.0 |  | 0.4 | 2.0
Proceeds from Sale of Equity Method Investments |  |  | 0.0 |  | 0.0 | 0.0
Closing value of investment, ending balance |  |  |  |  | 7.6 | 6.2
Fair Value, Recurring [Member] |  |  |  |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |
Investments, fair value |  |  | 115.0 |  | 117.1 | 134.6
Equity Method Investment, Nonconsolidated Investee, Other |  |  |  |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |
Unrealized Gain (Loss) on Investments |  |  | 0.5 |  | 1.0 | 0.3
MVI |  |  |  |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |
Opening undistributed value of investment, beginning balance |  | 1.0 | 1.0 | 0.8 | 0.8 | 0.5
Investment in the period | $ 0.8 |  | 0.0 | 0.4 | 0.0 | 0.4
Proceeds from Sale of Equity Method Investments |  |  | 0.0 |  | 0.0 | 0.0
Closing value of investment, ending balance |  |  | 1.1 |  | 1.0 | 0.8
MVI | Equity Method Investment, Nonconsolidated Investee, Other |  |  |  |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |
Unrealized Gain (Loss) on Investments |  |  | 0.1 |  | 0.2 | (0.1)
Multi-Line Insurer |  |  |  |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |
Opening undistributed value of investment, beginning balance |  | 6.4 | 6.4 | 5.2 | 5.2 | 3.2
Investment in the period |  | 0.0 | 0.0 |  | 0.4 | 1.6
Proceeds from Sale of Equity Method Investments |  |  | 0.0 |  | 0.0 | 0.0
Closing value of investment, ending balance |  |  | 6.8 |  | 6.4 | 5.2
Multi-Line Insurer | Equity Method Investment, Nonconsolidated Investee, Other |  |  |  |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |
Unrealized Gain (Loss) on Investments |  |  | 0.4 |  | 0.8 | 0.4
Digital Re |  |  |  |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |
Opening undistributed value of investment, beginning balance |  | $ 0.2 | 0.2 | $ 0.2 | 0.2 | 0.2
Investment in the period |  |  | 0.0 |  | 0.0 | 0.0
Proceeds from Sale of Equity Method Investments |  |  | 0.0 |  | 0.0 | 0.0
Closing value of investment, ending balance |  |  | 0.2 |  | 0.2 | 0.2
Digital Re | Equity Method Investment, Nonconsolidated Investee, Other |  |  |  |  |  |
Equity Method Investment, Aggregate Cost [Roll Forward] |  |  |  |  |  |
Unrealized Gain (Loss) on Investments |  |  | $ 0.0 |  | $ 0.0 | $ 0.0

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