v3.26.1
Commitments and Contingencies - Additional Information (Details)
£ in Millions, SFr in Millions, $ in Millions, $ in Millions, $ in Millions, $ in Millions
12 Months Ended
Dec. 31, 2025
USD ($)
Investment
Dec. 31, 2024
USD ($)
Investment
Dec. 31, 2023
USD ($)
Dec. 31, 2025
GBP (£)
Investment
Dec. 31, 2025
CAD ($)
Investment
Dec. 31, 2025
AUD ($)
Investment
Dec. 31, 2025
CHF (SFr)
Investment
Dec. 31, 2025
SGD ($)
Investment
Dec. 31, 2024
GBP (£)
Investment
Dec. 31, 2024
CAD ($)
Investment
Dec. 31, 2024
AUD ($)
Investment
Dec. 31, 2024
CHF (SFr)
Investment
Dec. 31, 2024
SGD ($)
Investment
Sep. 30, 2021
USD ($)
Dec. 20, 2017
USD ($)
Restricted Cash And Collateral [Line Items]                              
Other investments (equity method) [1] $ 279.6 $ 267.2                          
Total as percent of invested assets 37.10% 46.40%                          
Investable assets held by the Company $ 8,300.0 $ 7,700.0                          
Commitment to invest in private assets 78.0 57.0 $ 99.0                        
Investable assets held by insurance regulators $ 461.7 471.9                          
Minimum capital required | £       £ 0.4         £ 0.4            
Percentage of reinsurance liabilities 100.00%     100.00% 100.00% 100.00% 100.00% 100.00%              
Reinsurance liabilities $ 497.6 648.8                          
Regulatory deposits 3.4 6.9                          
Deposit with states $ 7.1 $ 6.5                          
Number of investments | Investment 1 1   1 1 1 1 1 1 1 1 1 1    
Collect Premium Issue | Pending Litigation                              
Restricted Cash And Collateral [Line Items]                              
Loss Contingency, Damages Sought, Value $ 150.0                            
Straddle Account Issue | Pending Litigation                              
Restricted Cash And Collateral [Line Items]                              
Loss Contingency, Damages Sought, Value 50.0                            
Construction Defect Issue | Pending Litigation                              
Restricted Cash And Collateral [Line Items]                              
Loss Contingency, Damages Sought, Value 100.0                            
U.S. Reinsurance Trust Fund                              
Restricted Cash And Collateral [Line Items]                              
Assets held-in-trust 718.6 $ 1,001.5                          
Us Multi Beneficiary Trust Fund                              
Restricted Cash And Collateral [Line Items]                              
Assets held-in-trust 243.8 334.0                          
US Surplus Lines Trust Fund                              
Restricted Cash And Collateral [Line Items]                              
Assets held-in-trust 155.2 150.2                          
Canadian Trust Fund                              
Restricted Cash And Collateral [Line Items]                              
Assets held-in-trust 176.7       $ 242.3         $ 219.8          
Australian Trust Fund                              
Restricted Cash And Collateral [Line Items]                              
Assets held-in-trust 42.5         $ 63.7         $ 78.1        
Swiss Trust Fund                              
Restricted Cash And Collateral [Line Items]                              
Assets held-in-trust 6.5           SFr 5.1         SFr 4.8      
Singapore Trust Fund                              
Restricted Cash And Collateral [Line Items]                              
Assets held-in-trust 165.8             $ 213.2         $ 201.7    
US Mortgage Trusts                              
Restricted Cash And Collateral [Line Items]                              
Assets held-in-trust $ 547.4 439.2                          
Bermuda                              
Restricted Cash And Collateral [Line Items]                              
Percentage of reinsurance liabilities 100.00%     100.00% 100.00% 100.00% 100.00% 100.00%              
Reinsurance liabilities $ 151.3 $ 182.3                          
Bermuda | Us Multi Beneficiary Trust Fund                              
Restricted Cash And Collateral [Line Items]                              
Minimum trust fund 20.0                            
Aspen UK | U.S. Reinsurance Trust Fund                              
Restricted Cash And Collateral [Line Items]                              
Minimum trust fund $ 20.0                            
Limited Partner                              
Restricted Cash And Collateral [Line Items]                              
Restricted assets                           $ 20.0 $ 100.0
[1] Other investments includes related party investments of $81.4 million (2024 — $78.6 million).