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Income Taxes - Additional Information (Details) - USD ($)
$ in Millions
 12 Months Ended
Apr. 01, 2023
Dec. 31, 2024
Dec. 31, 2023
Income Taxes [Line Items]      
Capital loss carryforwards   $ 17.8 $ 9.7
Less valuation allowance   64.0 172.7
Deferred Tax Assets, Gross   $ 505.8 529.9
Bermuda      
Income Taxes [Line Items]      
Corporate tax rate   0.00%  
Internal Revenue Service (IRS)      
Income Taxes [Line Items]      
Corporate tax rate   21.00%  
Net operating loss carryforwards   $ 324.8 354.9
Operating loss carryforwards, remaining   324.8  
Capital loss carryforwards   84.7  
Less valuation allowance   22.9 $ 24.5
Internal Revenue Service (IRS) | Aspen UK      
Income Taxes [Line Items]      
Capital loss carryforwards   33.1  
Internal Revenue Service (IRS) | U.S. operating subsidiaries      
Income Taxes [Line Items]      
Net operating loss carryforwards   247.2  
Capital loss carryforwards   51.6  
Operating loss carryforwards, subject to s382 limitation   20.8  
Internal Revenue Service (IRS) | Aspen UK's U.S. branch      
Income Taxes [Line Items]      
Net operating loss carryforwards   77.6  
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]      
Corporate tax rate 25.00% 25.00% 19.00%
Net operating loss carryforwards   $ 249.5 $ 248.1
Operating loss carryforwards, remaining   22.0 19.0
Less valuation allowance   26.1 131.0
U.K. | Lloyd's      
Income Taxes [Line Items]      
Deferred tax losses, operating loss carryforward   (29.3) 64.5
U.K. | Other juridictions      
Income Taxes [Line Items]      
Deferred tax losses, operating loss carryforward   $ (86.1) $ (97.8)