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Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($)
$ in Millions
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Schedule of Available-for-sale Securities [Line Items]      
Allowance for Credit Losses $ (8.1) $ (5.7) $ (10.0)
Amortized Cost, Net of Allowance for Credit Losses 9,685.7 9,051.5  
Fair Value 9,536.5 8,542.2  
U.S. Treasury and Government Agencies [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Fixed maturities, amortized cost 491.3 552.6  
Allowance for Credit Losses 0.0 0.0  
Amortized Cost, Net of Allowance for Credit Losses 491.3 552.6  
Gross Unrealized Gains 2.4 0.4  
Gross Unrealized Loss 38.2 58.8  
Fair Value 455.5 494.2  
Foreign Governments [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Fixed maturities, amortized cost 2.5 1.8  
Allowance for Credit Losses 0.0 0.0  
Amortized Cost, Net of Allowance for Credit Losses 2.5 1.8  
Gross Unrealized Gains 0.2 0.1  
Gross Unrealized Loss 0.0 0.0  
Fair Value 2.7 1.9  
Municipals [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Fixed maturities, amortized cost 910.8 1,001.5  
Allowance for Credit Losses 0.0 0.0  
Amortized Cost, Net of Allowance for Credit Losses 910.8 1,001.5  
Gross Unrealized Gains 4.1 1.5  
Gross Unrealized Loss 67.2 109.2  
Fair Value 847.7 893.8  
Corporates [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Fixed maturities, amortized cost 4,239.3 3,953.5  
Allowance for Credit Losses (0.2) (0.6)  
Amortized Cost, Net of Allowance for Credit Losses 4,239.1 3,952.9  
Gross Unrealized Gains 71.4 14.4  
Gross Unrealized Loss 57.0 161.4  
Fair Value 4,253.5 3,805.9  
Residential Mortgage-Backed [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Fixed maturities, amortized cost 2,472.2 2,277.6  
Allowance for Credit Losses 0.0 0.0  
Amortized Cost, Net of Allowance for Credit Losses 2,472.2 2,277.6  
Gross Unrealized Gains 32.4 2.8  
Gross Unrealized Loss 82.8 156.3  
Fair Value 2,421.8 2,124.1  
Commercial Mortgage-Backed [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Fixed maturities, amortized cost 610.2 564.2  
Allowance for Credit Losses 0.0 0.0  
Amortized Cost, Net of Allowance for Credit Losses 610.2 564.2  
Gross Unrealized Gains 1.0 0.1  
Gross Unrealized Loss 21.3 42.1  
Fair Value 589.9 522.2  
Other Asset-Backed [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Fixed maturities, amortized cost 959.6 700.9  
Allowance for Credit Losses 0.0 0.0  
Amortized Cost, Net of Allowance for Credit Losses 959.6 700.9  
Gross Unrealized Gains 6.5 2.5  
Gross Unrealized Loss 0.7 3.3  
Fair Value 965.4 700.1  
Fixed Maturities Including Held For Sale [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Fixed maturities, amortized cost 9,685.9 9,052.1  
Allowance for Credit Losses (0.2) (0.6)  
Amortized Cost, Net of Allowance for Credit Losses 9,685.7 9,051.5  
Gross Unrealized Gains 118.0 21.8  
Gross Unrealized Loss 267.2 531.1  
Fair Value $ 9,536.5 $ 8,542.2