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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%