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Income Taxes - Additional Information (Details) - USD ($)
$ in Millions
 12 Months Ended
Apr. 01, 2023
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2025
Dec. 31, 2024
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 25.00% 23.50% 19.00%    
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%