v3.26.1
Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Mar. 31, 2026
Dec. 31, 2025
Jun. 30, 2025
Mar. 31, 2025
Dec. 31, 2024
Schedule of Available-for-sale Securities [Line Items]            
Allowance for Credit Losses $ (9.0) $ (8.5) $ (8.1) $ (8.0) $ (5.7) $ (5.7)
Amortized Cost, Net of Allowance for Credit Losses 10,320.6   9,685.7      
Fair Value 10,061.1   9,536.5      
U.S. Treasury and Government Agencies [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 476.7   491.3      
Allowance for Credit Losses 0.0   0.0      
Amortized Cost, Net of Allowance for Credit Losses 476.7   491.3      
Gross Unrealized Gains 0.6   2.4      
Gross Unrealized Loss 42.4   38.2      
Fair Value 434.9   455.5      
Foreign Governments [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 17.5   2.5      
Allowance for Credit Losses 0.0   0.0      
Amortized Cost, Net of Allowance for Credit Losses 17.5   2.5      
Gross Unrealized Gains 0.1   0.2      
Gross Unrealized Loss 0.3   0.0      
Fair Value 17.3   2.7      
Municipals [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 1,072.8   910.8      
Allowance for Credit Losses 0.0   0.0      
Amortized Cost, Net of Allowance for Credit Losses 1,072.8   910.8      
Gross Unrealized Gains 2.2   4.1      
Gross Unrealized Loss 71.9   67.2      
Fair Value 1,003.1   847.7      
Corporates [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 4,560.6   4,239.3      
Allowance for Credit Losses (0.3)   (0.2)      
Amortized Cost, Net of Allowance for Credit Losses 4,560.3   4,239.1      
Gross Unrealized Gains 29.1   71.4      
Gross Unrealized Loss 74.2   57.0      
Fair Value 4,515.2   4,253.5      
Residential Mortgage-Backed [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 2,308.1   2,472.2      
Allowance for Credit Losses 0.0   0.0      
Amortized Cost, Net of Allowance for Credit Losses 2,308.1   2,472.2      
Gross Unrealized Gains 17.2   32.4      
Gross Unrealized Loss 89.3   82.8      
Fair Value 2,236.0   2,421.8      
Commercial Mortgage-Backed [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 829.9   610.2      
Allowance for Credit Losses 0.0   0.0      
Amortized Cost, Net of Allowance for Credit Losses 829.9   610.2      
Gross Unrealized Gains 0.0   1.0      
Gross Unrealized Loss 27.9   21.3      
Fair Value 802.0   589.9      
Other Asset-Backed [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 1,055.3   959.6      
Allowance for Credit Losses 0.0   0.0      
Amortized Cost, Net of Allowance for Credit Losses 1,055.3   959.6      
Gross Unrealized Gains 2.0   6.5      
Gross Unrealized Loss 4.7   0.7      
Fair Value 1,052.6   965.4      
Fixed Maturities Including Held For Sale [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 10,320.9   9,685.9      
Allowance for Credit Losses (0.3)   (0.2)      
Amortized Cost, Net of Allowance for Credit Losses 10,320.6   9,685.7      
Gross Unrealized Gains 51.2   118.0      
Gross Unrealized Loss 310.7   267.2      
Fair Value $ 10,061.1   $ 9,536.5