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Significant Transactions - Schedule of Assets Acquired and Liabilities Assumed, Bamboo (Details) - USD ($) $ in Millions | Jan. 02, 2024 | Sep. 30, 2024 | Jun. 30, 2024 | Dec. 31, 2023 | Sep. 30, 2023 | Jun. 30, 2023 | Dec. 31, 2022
--- | --- | --- | --- | --- | --- | --- | ---
Other Significant Transactions [Line Items] |  |  |  |  |  |  |
Goodwill |  | $ 439.2 | $ 439.2 | $ 168.8 | $ 168.8 | $ 168.8 | $ 176.5
Bamboo Ide8 Insurance Services LLC |  |  |  |  |  |  |
Other Significant Transactions [Line Items] |  |  |  |  |  |  |
Cash contributions for transaction | $ 20.0 |  |  |  |  |  |
Bamboo Ide8 Insurance Services LLC |  |  |  |  |  |  |
Other Significant Transactions [Line Items] |  |  |  |  |  |  |
Fixed maturity investments, at fair value | 8.2 |  |  |  |  |  |
Short-term investments, at fair value | 9.3 |  |  |  |  |  |
Restricted cash | 37.0 |  |  |  |  |  |
Cash (restricted $37.0) | 44.9 |  |  |  |  |  |
Premiums and commissions receivable | 38.0 |  |  |  |  |  |
Other assets | 7.7 |  |  |  |  |  |
Total tangible assets | 108.1 |  |  |  |  |  |
Loss and loss adjustment expense reserves | (9.3) |  |  |  |  |  |
Unearned insurance premiums | (20.1) |  |  |  |  |  |
Premiums and commissions payable | (49.2) |  |  |  |  |  |
Other liabilities | (13.1) |  |  |  |  |  |
Total tangible liabilities | (91.7) |  |  |  |  |  |
Net tangible assets acquired | 16.4 |  |  |  |  |  |
Goodwill | 270.4 |  |  |  |  |  |
Other intangible assets | 101.0 |  |  |  |  |  |
Total goodwill and other intangible assets | 371.4 |  |  |  |  |  |
Net assets acquired | $ 387.8 |  |  |  |  |  |

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