v3.26.3
Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses $ 307.1 $ 5.3
12 months or less, Fair Value 7,327.1 712.4
Greater than 12 months, Gross Unrealized Losses 335.7 261.9
Greater than 12 months, Fair Value 1,919.3 2,444.2
Total, Gross Unrealized Losses 642.8 267.2
Total, Fair Value 9,246.4 3,156.6
Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 297.2 3.1
12 months or less, Fair Value 7,070.1 673.6
Greater than 12 months, Gross Unrealized Losses 329.7 258.5
Greater than 12 months, Fair Value 1,881.5 2,404.5
Total, Gross Unrealized Losses 626.9 261.6
Total, Fair Value 8,951.6 3,078.1
Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 9.9 2.2
12 months or less, Fair Value 257.0 38.8
Greater than 12 months, Gross Unrealized Losses 6.0 3.4
Greater than 12 months, Fair Value 37.8 39.7
Total, Gross Unrealized Losses 15.9 5.6
Total, Fair Value 294.8 78.5
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 3.8 0.0
12 months or less, Fair Value 202.4 11.9
Greater than 12 months, Gross Unrealized Losses 55.5 38.2
Greater than 12 months, Fair Value 289.2 319.7
Total, Gross Unrealized Losses 59.3 38.2
Total, Fair Value 491.6 331.6
Foreign Governments [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 1.1  
12 months or less, Fair Value 13.8  
Greater than 12 months, Gross Unrealized Losses 0.0  
Greater than 12 months, Fair Value 0.0  
Total, Gross Unrealized Losses 1.1  
Total, Fair Value 13.8  
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 0.2  
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 0.2  
Municipals [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 28.9 0.6
12 months or less, Fair Value 495.4 56.3
Greater than 12 months, Gross Unrealized Losses 83.1 66.6
Greater than 12 months, Fair Value 470.8 550.3
Total, Gross Unrealized Losses 112.0 67.2
Total, Fair Value 966.2 606.6
Corporates [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 138.9 1.0
12 months or less, Fair Value 3,284.1 271.3
Greater than 12 months, Gross Unrealized Losses 62.7 51.2
Greater than 12 months, Fair Value 534.2 704.2
Total, Gross Unrealized Losses 201.6 52.2
Total, Fair Value 3,818.3 975.5
Corporates [Member] | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 9.4 2.2
12 months or less, Fair Value 251.5 37.5
Greater than 12 months, Gross Unrealized Losses 5.0 2.6
Greater than 12 months, Fair Value 33.3 34.8
Total, Gross Unrealized Losses 14.4 4.8
Total, Fair Value 284.8 72.3
Residential Mortgage-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 78.4 0.5
12 months or less, Fair Value 1,662.4 128.7
Greater than 12 months, Gross Unrealized Losses 108.6 82.3
Greater than 12 months, Fair Value 433.9 521.7
Total, Gross Unrealized Losses 187.0 82.8
Total, Fair Value 2,096.3 650.4
Residential Mortgage-Backed [Member] | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.5  
12 months or less, Fair Value 2.4  
Greater than 12 months, Gross Unrealized Losses 0.0  
Greater than 12 months, Fair Value 0.0  
Total, Gross Unrealized Losses 0.5  
Total, Fair Value 2.4  
Commercial Mortgage-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 26.6 0.5
12 months or less, Fair Value 463.8 122.5
Greater than 12 months, Gross Unrealized Losses 19.2 20.0
Greater than 12 months, Fair Value 148.7 284.3
Total, Gross Unrealized Losses 45.8 20.5
Total, Fair Value 612.5 406.8
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 1.0 0.8
Greater than 12 months, Fair Value 4.5 4.9
Total, Gross Unrealized Losses 1.0 0.8
Total, Fair Value 4.5 4.9
Other Asset-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 19.5 0.5
12 months or less, Fair Value 948.2 82.9
Greater than 12 months, Gross Unrealized Losses 0.6 0.2
Greater than 12 months, Fair Value 4.7 24.3
Total, Gross Unrealized Losses 20.1 0.7
Total, Fair Value 952.9 107.2
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 0.0
12 months or less, Fair Value 2.9 1.3
Greater than 12 months, Gross Unrealized Losses 0.0 0.0
Greater than 12 months, Fair Value 0.0 0.0
Total, Gross Unrealized Losses 0.0 0.0
Total, Fair Value $ 2.9 $ 1.3