v3.26.3
Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($)
$ in Millions
Sep. 30, 2026
Jun. 30, 2026
Dec. 31, 2025
Sep. 30, 2025
Jun. 30, 2025
Dec. 31, 2024
Schedule of Available-for-sale Securities [Line Items]            
Allowance for Credit Losses $ (7.0) $ (9.0) $ (8.1) $ (8.2) $ (8.0) $ (5.7)
Amortized Cost, Net of Allowance for Credit Losses 10,471.8   9,685.7      
Fair Value 9,835.7   9,536.5      
U.S. Treasury and Government Agencies [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 552.2   491.3      
Allowance for Credit Losses 0.0   0.0      
Amortized Cost, Net of Allowance for Credit Losses 552.2   491.3      
Gross Unrealized Gains 0.0   2.4      
Gross Unrealized Loss 59.3   38.2      
Fair Value 492.9   455.5      
Foreign Governments [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 16.6   2.5      
Allowance for Credit Losses 0.0   0.0      
Amortized Cost, Net of Allowance for Credit Losses 16.6   2.5      
Gross Unrealized Gains 0.1   0.2      
Gross Unrealized Loss 1.1   0.0      
Fair Value 15.6   2.7      
Municipals [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 1,100.8   910.8      
Allowance for Credit Losses 0.0   0.0      
Amortized Cost, Net of Allowance for Credit Losses 1,100.8   910.8      
Gross Unrealized Gains 0.1   4.1      
Gross Unrealized Loss 112.0   67.2      
Fair Value 988.9   847.7      
Corporates [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 4,629.2   4,239.3      
Allowance for Credit Losses (0.7)   (0.2)      
Amortized Cost, Net of Allowance for Credit Losses 4,628.5   4,239.1      
Gross Unrealized Gains 4.2   71.4      
Gross Unrealized Loss 216.0   57.0      
Fair Value 4,416.7   4,253.5      
Residential Mortgage-Backed [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 2,340.8   2,472.2      
Allowance for Credit Losses 0.0   0.0      
Amortized Cost, Net of Allowance for Credit Losses 2,340.8   2,472.2      
Gross Unrealized Gains 2.0   32.4      
Gross Unrealized Loss 187.5   82.8      
Fair Value 2,155.3   2,421.8      
Commercial Mortgage-Backed [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 663.8   610.2      
Allowance for Credit Losses 0.0   0.0      
Amortized Cost, Net of Allowance for Credit Losses 663.8   610.2      
Gross Unrealized Gains 0.0   1.0      
Gross Unrealized Loss 46.8   21.3      
Fair Value 617.0   589.9      
Other Asset-Backed [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 1,169.1   959.6      
Allowance for Credit Losses 0.0   0.0      
Amortized Cost, Net of Allowance for Credit Losses 1,169.1   959.6      
Gross Unrealized Gains 0.3   6.5      
Gross Unrealized Loss 20.1   0.7      
Fair Value 1,149.3   965.4      
Fixed Maturities Including Held For Sale [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 10,472.5   9,685.9      
Allowance for Credit Losses (0.7)   (0.2)      
Amortized Cost, Net of Allowance for Credit Losses 10,471.8   9,685.7      
Gross Unrealized Gains 6.7   118.0      
Gross Unrealized Loss 642.8   267.2      
Fair Value $ 9,835.7   $ 9,536.5