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Investments (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Schedule of Investments [Line Items]    
Other investments $ 661.4 $ 709.9
Concentration of investment in a single investee, maximum 10.00% 10.00%
Contractual investment commitments $ 283.7  
U.S. Government    
Schedule of Investments [Line Items]    
Marketable securities, fixed maturities   $ 304.9
Mortgage Loan Participations [Member]    
Schedule of Investments [Line Items]    
Due in current fiscal year, amortized cost 59.6  
Due in year one, amortized cost 29.4  
Due in year two, amortized cost 20.8  
Due in year three, amortized cost 46.4  
Due in year four and thereafter, amortized cost 90.8  
U.S. Treasury and Government Agencies [Member]    
Schedule of Investments [Line Items]    
Securities on loan, fair value 356.4 316.3
Mortgage Loans on Real Estate [Member]    
Schedule of Investments [Line Items]    
Other investments 247.0 304.9
Limited Partnership [Member]    
Schedule of Investments [Line Items]    
Other investments 391.5 395.0
Federal Home Loan Bank of Boston [Member]    
Schedule of Investments [Line Items]    
Securities on loan, fair value $ 339.2 $ 130.9