v3.26.1
Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses $ 45.0 $ 5.3
12 months or less, Fair Value 4,000.2 712.4
Greater than 12 months, Gross Unrealized Losses 265.7 261.9
Greater than 12 months, Fair Value 2,102.4 2,444.2
Total, Gross Unrealized Losses 310.7 267.2
Total, Fair Value 6,102.6 3,156.6
Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 41.9 3.1
12 months or less, Fair Value 3,866.2 673.6
Greater than 12 months, Gross Unrealized Losses 261.8 258.5
Greater than 12 months, Fair Value 2,067.2 2,404.5
Total, Gross Unrealized Losses 303.7 261.6
Total, Fair Value 5,933.4 3,078.1
Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 3.1 2.2
12 months or less, Fair Value 134.0 38.8
Greater than 12 months, Gross Unrealized Losses 3.9 3.4
Greater than 12 months, Fair Value 35.2 39.7
Total, Gross Unrealized Losses 7.0 5.6
Total, Fair Value 169.2 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 0.5 0.0
12 months or less, Fair Value 60.4 11.9
Greater than 12 months, Gross Unrealized Losses 41.9 38.2
Greater than 12 months, Fair Value 304.6 319.7
Total, Gross Unrealized Losses 42.4 38.2
Total, Fair Value 365.0 331.6
Foreign Governments [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.3  
12 months or less, Fair Value 14.6  
Total, Gross Unrealized Losses 0.3  
Total, Fair Value 14.6  
Foreign Governments [Member] | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Fair Value 0.7  
Total, Fair Value 0.7  
Municipals [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 5.0 0.6
12 months or less, Fair Value 317.6 56.3
Greater than 12 months, Gross Unrealized Losses 66.9 66.6
Greater than 12 months, Fair Value 495.5 550.3
Total, Gross Unrealized Losses 71.9 67.2
Total, Fair Value 813.1 606.6
Corporates [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 18.2 1.0
12 months or less, Fair Value 1,639.4 271.3
Greater than 12 months, Gross Unrealized Losses 49.8 51.2
Greater than 12 months, Fair Value 554.1 704.2
Total, Gross Unrealized Losses 68.0 52.2
Total, Fair Value 2,193.5 975.5
Corporates [Member] | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 3.1 2.2
12 months or less, Fair Value 130.0 37.5
Greater than 12 months, Gross Unrealized Losses 3.1 2.6
Greater than 12 months, Fair Value 30.5 34.8
Total, Gross Unrealized Losses 6.2 4.8
Total, Fair Value 160.5 72.3
Residential Mortgage-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 4.9 0.5
12 months or less, Fair Value 703.5 128.7
Greater than 12 months, Gross Unrealized Losses 84.4 82.3
Greater than 12 months, Fair Value 486.3 521.7
Total, Gross Unrealized Losses 89.3 82.8
Total, Fair Value 1,189.8 650.4
Residential Mortgage-Backed [Member] | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Fair Value 0.9  
Total, Fair Value 0.9  
Commercial Mortgage-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 8.6 0.5
12 months or less, Fair Value 568.2 122.5
Greater than 12 months, Gross Unrealized Losses 18.5 20.0
Greater than 12 months, Fair Value 221.5 284.3
Total, Gross Unrealized Losses 27.1 20.5
Total, Fair Value 789.7 406.8
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 0.8 0.8
Greater than 12 months, Fair Value 4.7 4.9
Total, Gross Unrealized Losses 0.8 0.8
Total, Fair Value 4.7 4.9
Other Asset-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 4.4 0.5
12 months or less, Fair Value 562.5 82.9
Greater than 12 months, Gross Unrealized Losses 0.3 0.2
Greater than 12 months, Fair Value 5.2 24.3
Total, Gross Unrealized Losses 4.7 0.7
Total, Fair Value 567.7 107.2
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 2.4 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 $ 2.4 $ 1.3