v3.26.1
Investments - Additional Information (Narrative) (Details)
£ in Millions
12 Months Ended
Dec. 31, 2025
USD ($)
security
Dec. 31, 2024
USD ($)
security
Dec. 31, 2023
USD ($)
Dec. 31, 2025
GBP (£)
security
Dec. 31, 2024
GBP (£)
security
Aug. 01, 2024
USD ($)
Sep. 30, 2023
USD ($)
Aug. 31, 2023
GBP (£)
Apr. 01, 2023
USD ($)
May 05, 2022
USD ($)
Apr. 01, 2022
USD ($)
Sep. 30, 2021
USD ($)
Dec. 31, 2020
USD ($)
Dec. 20, 2017
USD ($)
Gain (Loss) on Securities [Line Items]                            
Catastrophe bonds, trading at fair value (amortized cost — 2025: $0.0 and 2024: $1.0) $ 0 $ 1,000,000.0                        
Fair Market Value 1,162,000,000 1,488,700,000                        
Unrealized (loss)/gain for the twelve months to December 31, 2024 0 0 $ 400,000                      
Other Investments [1] 279,600,000 $ 267,200,000                        
Investment funded $ 0                          
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions (606) (1,170)   (606) (1,170)                  
Total, Gross Unrealized Losses $ (58,200,000) $ (181,900,000)                        
Securities Below Investment Grade or NR (300,000)                          
Investment, Identifier [Axis]: Unfunded Commitment, Real Estate (2021)Fund                            
Gain (Loss) on Securities [Line Items]                            
Unfunded commitment 900,000 900,000                        
Investment, Identifier [Axis]: Unfunded Commitment, Apollo Real Estate Fund                            
Gain (Loss) on Securities [Line Items]                            
Unfunded commitment 3,700,000 3,700,000                        
Investment, Identifier [Axis]: Unfunded Commitment, Energy Fund                            
Gain (Loss) on Securities [Line Items]                            
Unfunded commitment 27,200,000 55,000,000.0                        
Investment, Identifier [Axis]: Unfunded Commitment, Infrastructure fund                            
Gain (Loss) on Securities [Line Items]                            
Unfunded commitment 1,700,000 2,300,000                        
Investment, Identifier [Axis]: Unfunded Commitment, Lloyd's Enhanced Liquidity Fund                            
Gain (Loss) on Securities [Line Items]                            
Unfunded commitment 0 0                        
Investment, Identifier [Axis]: Unfunded Commitment, Managed Debt Fund                            
Gain (Loss) on Securities [Line Items]                            
Unfunded commitment | £       £ 5.7 £ 6.8                  
Investment, Identifier [Axis]: Unfunded Commitment, Managed lending fund                            
Gain (Loss) on Securities [Line Items]                            
Unfunded commitment   200,000                        
Fair Value, Recurring [Member]                            
Gain (Loss) on Securities [Line Items]                            
Investments, fair value 88,300,000 111,700,000                        
Fair Value, Recurring [Member] | Managed Lending Fund (Dec 2020)                            
Gain (Loss) on Securities [Line Items]                            
Investments, fair value 20,700,000 19,300,000                        
Fair Value, Recurring [Member] | Pledge Accounts, Custodian Bank (April 2021)                            
Gain (Loss) on Securities [Line Items]                            
Investments, fair value 2,200,000 2,000,000.0                        
Fair Value, Recurring [Member] | Real estate fund (Sept 2021)                            
Gain (Loss) on Securities [Line Items]                            
Investments, fair value 31,100,000 36,200,000                 $ 10,000,000.0      
Fair Value, Recurring [Member] | Real estate fund                            
Gain (Loss) on Securities [Line Items]                            
Investments, fair value 279,600,000 267,200,000                        
Fair Value, Recurring [Member] | Apollo real estate fund                            
Gain (Loss) on Securities [Line Items]                            
Investments, fair value 15,000,000.0 19,300,000                        
Fair Value, Recurring [Member] | Infrastructure fund                            
Gain (Loss) on Securities [Line Items]                            
Investments, fair value 15,500,000 12,900,000                        
Fair Value, Recurring [Member] | Managed Debt Fund                            
Gain (Loss) on Securities [Line Items]                            
Investments, fair value 1,600,000 100,000                        
Fair Value, Recurring [Member] | Third party energy fund                            
Gain (Loss) on Securities [Line Items]                            
Investments, fair value 32,500,000                          
Fair Value, Recurring [Member] | Lloyd's enhanced liquidity fund                            
Gain (Loss) on Securities [Line Items]                            
Investments, fair value 27,000,000.0 25,700,000                        
Fair Value, Recurring [Member] | Five Times Square                            
Gain (Loss) on Securities [Line Items]                            
Investments, fair value 43,200,000 40,000,000.0                        
Fair Value, Recurring [Member] | Five Times Square 2                            
Gain (Loss) on Securities [Line Items]                            
Investments, fair value 2,500,000                          
Fair Value, Recurring [Member] | Equity investments                            
Gain (Loss) on Securities [Line Items]                            
Investments, fair value 8,100,000 0                        
Limited Partner                            
Gain (Loss) on Securities [Line Items]                            
Restricted Assets                       $ 20,000,000.0   $ 100,000,000.0
Limited Partner | Managed lending fund                            
Gain (Loss) on Securities [Line Items]                            
Restricted Assets                 $ 2,800,000       $ 10,500,000  
Limited Partner | Real estate fund | Apollo real estate fund                            
Gain (Loss) on Securities [Line Items]                            
Restricted Assets                     $ 30,000,000.0      
Limited Partner | Infrastructure fund                            
Gain (Loss) on Securities [Line Items]                            
Restricted Assets                   $ 15,000,000.0        
Limited Partner | Managed Debt Fund                            
Gain (Loss) on Securities [Line Items]                            
Restricted Assets | £               £ 7.0            
Limited Partner | Third party energy fund                            
Gain (Loss) on Securities [Line Items]                            
Restricted Assets             $ 55,000,000.0              
Limited Partner | Lloyd's enhanced liquidity fund                            
Gain (Loss) on Securities [Line Items]                            
Restricted Assets           $ 25,000,000.0                
Commercial mortgage loans                            
Gain (Loss) on Securities [Line Items]                            
Fair Market Value 25,400,000 79,700,000                        
Middle market loans and other private debt                            
Gain (Loss) on Securities [Line Items]                            
Fair Market Value 28,900,000 61,000,000.0                        
Private asset backed securities                            
Gain (Loss) on Securities [Line Items]                            
Fair Market Value 83,800,000 151,800,000                        
Global corporate securities                            
Gain (Loss) on Securities [Line Items]                            
Fair Market Value $ 34,200,000 $ 18,500,000                        
[1] Other investments includes related party investments of $81.4 million (2024 — $78.6 million).