v3.26.1
Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($)
$ in Millions
Mar. 31, 2026
Dec. 31, 2025
Mar. 31, 2025
Dec. 31, 2024
Schedule of Available-for-sale Securities [Line Items]        
Allowance for Credit Losses $ (8.5) $ (8.1) $ (5.7) $ (5.7)
Amortized Cost, Net of Allowance for Credit Losses 10,211.8 9,685.7    
Fair Value 9,976.2 9,536.5    
U.S. Treasury and Government Agencies [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Fixed maturities, amortized cost 477.7 491.3    
Allowance for Credit Losses 0.0 0.0    
Amortized Cost, Net of Allowance for Credit Losses 477.7 491.3    
Gross Unrealized Gains 1.4 2.4    
Gross Unrealized Loss 40.4 38.2    
Fair Value 438.7 455.5    
Foreign Governments [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Fixed maturities, amortized cost 2.8 2.5    
Allowance for Credit Losses 0.0 0.0    
Amortized Cost, Net of Allowance for Credit Losses 2.8 2.5    
Gross Unrealized Gains 0.1 0.2    
Gross Unrealized Loss 0.0 0.0    
Fair Value 2.9 2.7    
Municipals [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Fixed maturities, amortized cost 959.0 910.8    
Allowance for Credit Losses 0.0 0.0    
Amortized Cost, Net of Allowance for Credit Losses 959.0 910.8    
Gross Unrealized Gains 2.7 4.1    
Gross Unrealized Loss 68.5 67.2    
Fair Value 893.2 847.7    
Corporates [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Fixed maturities, amortized cost 4,388.5 4,239.3    
Allowance for Credit Losses (0.3) (0.2)    
Amortized Cost, Net of Allowance for Credit Losses 4,388.2 4,239.1    
Gross Unrealized Gains 34.8 71.4    
Gross Unrealized Loss 74.9 57.0    
Fair Value 4,348.1 4,253.5    
Residential Mortgage-Backed [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Fixed maturities, amortized cost 2,389.8 2,472.2    
Allowance for Credit Losses 0.0 0.0    
Amortized Cost, Net of Allowance for Credit Losses 2,389.8 2,472.2    
Gross Unrealized Gains 20.1 32.4    
Gross Unrealized Loss 85.6 82.8    
Fair Value 2,324.3 2,421.8    
Commercial Mortgage-Backed [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Fixed maturities, amortized cost 880.2 610.2    
Allowance for Credit Losses 0.0 0.0    
Amortized Cost, Net of Allowance for Credit Losses 880.2 610.2    
Gross Unrealized Gains 0.3 1.0    
Gross Unrealized Loss 25.6 21.3    
Fair Value 854.9 589.9    
Other Asset-Backed [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Fixed maturities, amortized cost 1,114.1 959.6    
Allowance for Credit Losses 0.0 0.0    
Amortized Cost, Net of Allowance for Credit Losses 1,114.1 959.6    
Gross Unrealized Gains 3.5 6.5    
Gross Unrealized Loss 3.5 0.7    
Fair Value 1,114.1 965.4    
Fixed Maturities Including Held For Sale [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Fixed maturities, amortized cost 10,212.1 9,685.9    
Allowance for Credit Losses (0.3) (0.2)    
Amortized Cost, Net of Allowance for Credit Losses 10,211.8 9,685.7    
Gross Unrealized Gains 62.9 118.0    
Gross Unrealized Loss 298.5 267.2    
Fair Value $ 9,976.2 $ 9,536.5