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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, 2024
Dec. 31, 2023
Dec. 31, 2022
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years $ (455.3) $ (38.6) $ (167.4)
Insurance [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (451.0) (104.7) (142.9)
Insurance [Member] | Professional liability [Member] | U.S. [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years 111.8 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 (152.6) (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] | General liability [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (81.6)    
Insurance [Member] | General liability [Member] | U.S. [Member]      
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 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] | 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] | Property [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (75.0) (95.1) (48.1)
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 (58.3) (76.5)  
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)
Insurance [Member] | Programs [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (53.9)   (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 (51.4)   (31.7)
Insurance [Member] | Other products [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (90.0) (45.9) (23.7)
Reinsurance [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (12.3) 57.1 (26.1)
Reinsurance [Member] | Professional liability [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (17.1)    
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  
Reinsurance [Member] | Property [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (23.0) (21.6) (29.2)
Reinsurance [Member] | Workers' compensation [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (20.3) (14.9)  
Reinsurance [Member] | Premium adjustments      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years (4.0) 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)
Reinsurance [Member] | Public entity      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years 34.1 53.7  
Reinsurance [Member] | Other products [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years 18.0 (67.0) (27.1)
Other [Member] | Other underwriting [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]      
Incurred losses and loss adjustment expenses, prior accident years $ 8.0 $ 9.0 $ 1.6