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Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($)
$ in Millions
Dec. 31, 2025
Dec. 31, 2024
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses $ 5.3 $ 54.6
12 months or less, Fair Value 712.4 2,860.0
Greater than 12 months, Gross Unrealized Losses 261.9 476.5
Greater than 12 months, Fair Value 2,444.2 3,806.7
Total, Gross Unrealized Losses 267.2 531.1
Total, Fair Value 3,156.6 6,666.7
Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 3.1 53.3
12 months or less, Fair Value 673.6 2,789.1
Greater than 12 months, Gross Unrealized Losses 258.5 471.2
Greater than 12 months, Fair Value 2,404.5 3,756.1
Total, Gross Unrealized Losses 261.6 524.5
Total, Fair Value 3,078.1 6,545.2
Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 2.2 1.3
12 months or less, Fair Value 38.8 70.9
Greater than 12 months, Gross Unrealized Losses 3.4 5.3
Greater than 12 months, Fair Value 39.7 50.6
Total, Gross Unrealized Losses 5.6 6.6
Total, Fair Value 78.5 121.5
U.S. Treasury and Government Agencies [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.0 7.8
12 months or less, Fair Value 11.9 194.4
Greater than 12 months, Gross Unrealized Losses 38.2 51.0
Greater than 12 months, Fair Value 319.7 240.8
Total, Gross Unrealized Losses 38.2 58.8
Total, Fair Value 331.6 435.2
Foreign Governments [Member] | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses   0.0
12 months or less, Fair Value   0.1
Greater than 12 months, Gross Unrealized Losses   0.0
Greater than 12 months, Fair Value   0.0
Total, Gross Unrealized Losses   0.0
Total, Fair Value   0.1
Municipals [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.6 3.0
12 months or less, Fair Value 56.3 132.5
Greater than 12 months, Gross Unrealized Losses 66.6 106.2
Greater than 12 months, Fair Value 550.3 680.9
Total, Gross Unrealized Losses 67.2 109.2
Total, Fair Value 606.6 813.4
Corporates [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 1.0 22.8
12 months or less, Fair Value 271.3 1,360.3
Greater than 12 months, Gross Unrealized Losses 51.2 132.0
Greater than 12 months, Fair Value 704.2 1,523.3
Total, Gross Unrealized Losses 52.2 154.8
Total, Fair Value 975.5 2,883.6
Corporates [Member] | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 2.2 1.3
12 months or less, Fair Value 37.5 70.8
Greater than 12 months, Gross Unrealized Losses 2.6 5.3
Greater than 12 months, Fair Value 34.8 49.8
Total, Gross Unrealized Losses 4.8 6.6
Total, Fair Value 72.3 120.6
Residential Mortgage-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.5 16.5
12 months or less, Fair Value 128.7 896.8
Greater than 12 months, Gross Unrealized Losses 82.3 139.8
Greater than 12 months, Fair Value 521.7 747.2
Total, Gross Unrealized Losses 82.8 156.3
Total, Fair Value 650.4 1,644.0
Commercial Mortgage-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.5 0.7
12 months or less, Fair Value 122.5 23.7
Greater than 12 months, Gross Unrealized Losses 20.0 41.4
Greater than 12 months, Fair Value 284.3 485.4
Total, Gross Unrealized Losses 20.5 42.1
Total, Fair Value 406.8 509.1
Commercial Mortgage-Backed [Member] | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.0 0.0
12 months or less, Fair Value 0.0 0.0
Greater than 12 months, Gross Unrealized Losses 0.8 0.0
Greater than 12 months, Fair Value 4.9 0.8
Total, Gross Unrealized Losses 0.8 0.0
Total, Fair Value 4.9 0.8
Other Asset-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.5 2.5
12 months or less, Fair Value 82.9 181.4
Greater than 12 months, Gross Unrealized Losses 0.2 0.8
Greater than 12 months, Fair Value 24.3 78.5
Total, Gross Unrealized Losses 0.7 3.3
Total, Fair Value 107.2 $ 259.9
Other Asset-Backed [Member] | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.0  
12 months or less, Fair Value 1.3  
Greater than 12 months, Gross Unrealized Losses 0.0  
Greater than 12 months, Fair Value 0.0  
Total, Gross Unrealized Losses 0.0  
Total, Fair Value $ 1.3