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Income Taxes - Income Tax Reconciliation (Details) - USD ($)
$ in Millions
 12 Months Ended
Apr. 01, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Operating Loss Carryforwards [Line Items]        
Income tax benefit at statutory tax rate of zero percent   $ 0.0 $ 0.0 $ 0.0
Overseas statutory tax rates differential   16.8 (0.9) (3.8)
Base erosion and anti-abuse (BEAT) expense   2.3 6.1 4.3
Prior year adjustments   (2.9) 0.5 (25.0)
Change in valuation allowance (2)   (98.9) 9.6 40.8
Impact of unrecognized tax benefits   0.0 0.0 0.0
Australian non-resident withholding tax   1.5 0.6 1.0
Foreign exchange   (0.3) (1.5) 0.2
Non-deductible expenses   2.4 2.4 4.7
Impact of changes in statutory tax rates (4)   (5.7) (11.5) (3.8)
Effective Income Tax Rate Reconciliation, Tax Effect of OCI, Amount   6.7 0.0 0.0
Total tax expense/(benefit)   (78.1) $ 5.3 $ 18.4
U.S. operating subsidiaries        
Operating Loss Carryforwards [Line Items]        
Base erosion and anti-abuse (BEAT) expense   1.1    
Aspen U.K.        
Operating Loss Carryforwards [Line Items]        
Base erosion and anti-abuse (BEAT) expense   $ 1.2    
U.K.        
Operating Loss Carryforwards [Line Items]        
U. K. corporate tax rate   19.00%    
U.K. | Forecast        
Operating Loss Carryforwards [Line Items]        
U. K. corporate tax rate 25.00%      
Lloyd's syndicate        
Operating Loss Carryforwards [Line Items]        
Valuation allowance, associated losses   $ 0.0