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Unpaid Losses And Loss Adjustment Expenses (Prior Year Losses and Loss Adjustment Expenses Development by Product Line) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years $ (38.6) $ (167.4) $ (479.8)
Insurance [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (104.7) (142.9) (506.3)
Insurance [Member] | General liability [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years     (139.7)
Insurance [Member] | General liability [Member] | U.S. and Bermuda      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years 274.0    
Insurance [Member] | General liability [Member] | International      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (36.5)    
Insurance [Member] | General liability [Member] | Short-Duration Insurance Contracts, Accident Years, all other      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years   (20.5)  
Insurance [Member] | General liability [Member] | Short-Duration Insurance Contracts, Accident Years 2016 to 2019      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years   61.6  
Insurance [Member] | Professional liability [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years     (54.7)
Insurance [Member] | Professional liability [Member] | U.S. and Bermuda      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years 56.8    
Insurance [Member] | Professional liability [Member] | International      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (102.2)    
Insurance [Member] | Professional liability [Member] | Short-Duration Insurance Contracts, Accident Years 2018 and 2019      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years   121.0  
Insurance [Member] | Professional liability [Member] | Short-Duration Insurance Contracts, Accident Years, all other      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years   (91.1)  
Insurance [Member] | Property [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (95.1) (48.1) (96.5)
Insurance [Member] | Marine and energy [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (76.5)   (60.0)
Insurance [Member] | Personal Lines      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (42.8)    
Insurance [Member] | Workers' compensation [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (36.5) (62.1) (79.0)
Insurance [Member] | Other products [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (45.9) (23.7) (76.4)
Insurance [Member] | Programs [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years   (48.3)  
Insurance [Member] | Credit and surety [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years   (31.7)  
Reinsurance [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years 57.1 (26.1) 19.9
Reinsurance [Member] | General liability [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years 95.5   (19.2)
Reinsurance [Member] | Professional liability [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years     29.2
Reinsurance [Member] | Property [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (21.6) (29.2) 35.0
Reinsurance [Member] | Workers' compensation [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (14.9)    
Reinsurance [Member] | Other products [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (67.0) (27.1) (8.5)
Reinsurance [Member] | Premium adjustments      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years 11.4 53.1  
Reinsurance [Member] | Credit and surety [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years   (22.9) (16.6)
Reinsurance [Member] | Public entity      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years 53.7    
Other [Member] | Other underwriting [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years $ 9.0 $ 1.6 $ 6.6