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Income Taxes - Additional Information (Details) - USD ($)
$ in Millions
 12 Months Ended
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%