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Income Taxes - Additional Information (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2024 | Dec. 31, 2023 | Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2022
Income Taxes [Line Items] |  |  |  |  |
United States corporate tax at a rate | 21.00% |  |  |  |
Capital loss carryforwards |  | $ 9.7 |  |  | $ 6.7
Less valuation allowance |  | 172.7 |  |  | 145.7
Deferred Tax Assets, Gross |  | 529.9 |  |  | 322.2
Internal Revenue Service (IRS) |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Net operating loss carryforwards |  | 354.9 |  |  | 376.5
Operating loss carryforwards, remaining |  |  |  | $ 354.9 |
Capital loss carryforwards |  | 46.1 |  |  |
Less valuation allowance |  | 24.5 |  |  | 18.0
Internal Revenue Service (IRS) | Aspen U.K. |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Capital loss carryforwards |  | 30.3 |  |  |
Operating loss carryforwards, subject to s382 limitation |  | 39.2 |  |  |
Internal Revenue Service (IRS) | Aspen U.K. | Subsequent Event |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Operating loss carryforwards, subject to s382 limitation |  |  |  | 23.1 |
Internal Revenue Service (IRS) | U.S. operating subsidiaries |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Net operating loss carryforwards |  | 270.2 |  |  |
Capital loss carryforwards |  | 15.8 |  |  |
Internal Revenue Service (IRS) | U.S. operating subsidiaries | Subsequent Event |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Operating loss carryforwards, subject to s382 limitation |  |  | $ 20.8 |  |
Internal Revenue Service (IRS) | Aspen U.K.'s U.S. branch |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Net operating loss carryforwards |  | $ 84.7 |  |  |
Operating loss carryforwards, subject to s382 limitation |  |  |  | $ 6.5 |
Internal Revenue Service (IRS) | Minimum |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Operating loss carryforwards, date of expiration |  | 2032 |  |  |
Internal Revenue Service (IRS) | Maximum |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Operating loss carryforwards, date of expiration |  | 2041 |  |  |
U.K. |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
U. K. corporate tax rate | 23.50% |  |  |  |
Net operating loss carryforwards |  | $ 248.1 |  |  | 280.5
Operating loss carryforwards, remaining |  | 19.0 |  |  | 19.8
Less valuation allowance |  | 131.0 |  |  | 106.7
U.K. | Lloyd's |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Deferred tax losses, operating loss carryforward |  | 64.5 |  |  | (19.9)
U.K. | Other juridictions |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Deferred tax losses, operating loss carryforward |  | $ (97.8) |  |  | $ (118.8)
Bermuda |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Bermuda tax rate |  | 0.00% |  |  |

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