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Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($)
$ in Millions
Sep. 30, 2024
Jun. 30, 2024
Dec. 31, 2023
Sep. 30, 2023
Jun. 30, 2023
Dec. 31, 2022
Schedule of Available-for-sale Securities [Line Items]            
Allowance for Credit Losses $ (7.5) $ (8.6) $ (10.0) $ (8.5) $ (8.5) $ (3.2)
Amortized Cost, Net of Allowance for Credit Losses 8,936.5   8,573.9      
Fair Value 8,620.0   7,985.3      
U.S. Treasury and Government Agencies [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 616.8   512.9      
Amortized Cost, Net of Allowance for Credit Losses 616.8   512.9      
Gross Unrealized Gains 4.0   1.4      
Gross Unrealized Loss 44.0   51.7      
Fair Value 576.8   462.6      
Foreign Governments [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 1.7   2.2      
Amortized Cost, Net of Allowance for Credit Losses 1.7   2.2      
Gross Unrealized Gains 0.1          
Fair Value 1.8   2.2      
Municipals [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 1,135.0   1,309.3      
Amortized Cost, Net of Allowance for Credit Losses 1,135.0   1,309.3      
Gross Unrealized Gains 4.3   5.5      
Gross Unrealized Loss 90.0   129.6      
Fair Value 1,049.3   1,185.2      
Corporates [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 4,047.3   4,053.8      
Allowance for Credit Losses (0.6)   (1.9)      
Amortized Cost, Net of Allowance for Credit Losses 4,046.7   4,051.9      
Gross Unrealized Gains 46.1   22.9      
Gross Unrealized Loss 118.9   225.1      
Fair Value 3,973.9   3,849.7      
Residential Mortgage-Backed [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 1,813.2   1,435.6      
Amortized Cost, Net of Allowance for Credit Losses 1,813.2   1,435.6      
Gross Unrealized Gains 20.7   5.5      
Gross Unrealized Loss 105.1   130.2      
Fair Value 1,728.8   1,310.9      
Commercial Mortgage-Backed [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 621.9   890.3      
Amortized Cost, Net of Allowance for Credit Losses 621.9   890.3      
Gross Unrealized Gains 0.7   0.1      
Gross Unrealized Loss 36.4   72.2      
Fair Value 586.2   818.2      
Other Asset-Backed [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 701.2   371.7      
Amortized Cost, Net of Allowance for Credit Losses 701.2   371.7      
Gross Unrealized Gains 4.2   0.7      
Gross Unrealized Loss 2.2   15.9      
Fair Value 703.2   356.5      
Fixed Maturities Including Held For Sale [Member]            
Schedule of Available-for-sale Securities [Line Items]            
Fixed maturities, amortized cost 8,937.1   8,575.8      
Allowance for Credit Losses (0.6)   (1.9)      
Amortized Cost, Net of Allowance for Credit Losses 8,936.5   8,573.9      
Gross Unrealized Gains 80.1   36.1      
Gross Unrealized Loss 396.6   624.7      
Fair Value $ 8,620.0   $ 7,985.3