v3.26.1
Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($)
$ in Millions
Mar. 31, 2026
Dec. 31, 2025
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses $ 32.3 $ 5.3
12 months or less, Fair Value 3,191.8 712.4
Greater than 12 months, Gross Unrealized Losses 266.2 261.9
Greater than 12 months, Fair Value 2,223.7 2,444.2
Total, Gross Unrealized Losses 298.5 267.2
Total, Fair Value 5,415.5 3,156.6
Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 28.0 3.1
12 months or less, Fair Value 3,038.2 673.6
Greater than 12 months, Gross Unrealized Losses 261.0 258.5
Greater than 12 months, Fair Value 2,185.7 2,404.5
Total, Gross Unrealized Losses 289.0 261.6
Total, Fair Value 5,223.9 3,078.1
Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 4.3 2.2
12 months or less, Fair Value 153.6 38.8
Greater than 12 months, Gross Unrealized Losses 5.2 3.4
Greater than 12 months, Fair Value 38.0 39.7
Total, Gross Unrealized Losses 9.5 5.6
Total, Fair Value 191.6 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 0.2 0.0
12 months or less, Fair Value 20.9 11.9
Greater than 12 months, Gross Unrealized Losses 40.2 38.2
Greater than 12 months, Fair Value 307.1 319.7
Total, Gross Unrealized Losses 40.4 38.2
Total, Fair Value 328.0 331.6
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 2.9 0.6
12 months or less, Fair Value 195.2 56.3
Greater than 12 months, Gross Unrealized Losses 65.6 66.6
Greater than 12 months, Fair Value 511.4 550.3
Total, Gross Unrealized Losses 68.5 67.2
Total, Fair Value 706.6 606.6
Corporates [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 13.6 1.0
12 months or less, Fair Value 1,296.1 271.3
Greater than 12 months, Gross Unrealized Losses 52.7 51.2
Greater than 12 months, Fair Value 586.1 704.2
Total, Gross Unrealized Losses 66.3 52.2
Total, Fair Value 1,882.2 975.5
Corporates [Member] | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 4.2 2.2
12 months or less, Fair Value 149.9 37.5
Greater than 12 months, Gross Unrealized Losses 4.4 2.6
Greater than 12 months, Fair Value 33.2 34.8
Total, Gross Unrealized Losses 8.6 4.8
Total, Fair Value 183.1 72.3
Residential Mortgage-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 2.9 0.5
12 months or less, Fair Value 456.9 128.7
Greater than 12 months, Gross Unrealized Losses 82.7 82.3
Greater than 12 months, Fair Value 503.8 521.7
Total, Gross Unrealized Losses 85.6 82.8
Total, Fair Value 960.7 650.4
Residential Mortgage-Backed [Member] | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Fair Value 0.5  
Total, Fair Value 0.5  
Commercial Mortgage-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 5.3 0.5
12 months or less, Fair Value 525.9 122.5
Greater than 12 months, Gross Unrealized Losses 19.5 20.0
Greater than 12 months, Fair Value 259.9 284.3
Total, Gross Unrealized Losses 24.8 20.5
Total, Fair Value 785.8 406.8
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 4.8 4.9
Total, Gross Unrealized Losses 0.8 0.8
Total, Fair Value 4.8 4.9
Other Asset-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 3.1 0.5
12 months or less, Fair Value 543.2 82.9
Greater than 12 months, Gross Unrealized Losses 0.3 0.2
Greater than 12 months, Fair Value 17.4 24.3
Total, Gross Unrealized Losses 3.4 0.7
Total, Fair Value 560.6 107.2
Other Asset-Backed [Member] | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.1 0.0
12 months or less, Fair Value 2.5 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 0.1 0.0
Total, Fair Value $ 2.5 $ 1.3