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Investments - Summary of Investment Income (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Investment income |  | $ 329.5 | $ 287.3 | $ 198.4
Gain (loss) on investments | [1] | 52.6 | 75.9 | 5.0
Investment expenses |  | (11.5) | (11.6) | (10.3)
Net investment income | [2] | 318.0 | 275.7 | 188.1
Fixed income securities — Available for sale |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Investment income |  | 161.5 | 115.7 | 95.6
Fixed income securities — Trading |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Investment income |  | 88.2 | 98.9 | 57.0
Short-term investments — Available for sale |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Investment income |  | 4.5 | 5.3 | 0.6
Short-term investments — Trading |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Investment income |  | 0.1 | 0.3 | 0.1
Fixed term deposits (included in cash and cash equivalents) |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Investment income |  | 45.3 | 39.9 | 6.6
Catastrophe bonds — Trading |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Investment income |  | 0.0 | 0.2 | 0.3
Privately-held investments — Available for sale |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Investment income |  | 1.2 | 0.1 | 0.0
Privately-held investments — Trading |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Investment income |  | 34.9 | 44.7 | 24.3
Other investments, at fair value |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |
Gain (loss) on investments |  | $ 6.2 | $ 17.8 | $ 13.9
[1] | Realized and unrealized investments gains includes gains of $2.5 million on related party investments (2023 — gains of $8.7 million, 2022 — loss of $0.4 million).
[2] | Net investment income includes related party net investment income of $15.2 million (2023 — $19.6 million, 2022 — $3.1 million) and related party investment management fees of $9.2 million (2023 — $9.4 million, 2022 — $4.9 million).

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