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Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($)
$ in Millions
Sep. 30, 2024
Dec. 31, 2023
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses $ 2.3 $ 8.4
12 months or less, Fair Value 693.8 509.8
Greater than 12 months, Gross Unrealized Losses 394.3 616.3
Greater than 12 months, Fair Value 4,721.4 6,101.1
Total, Gross Unrealized Losses 396.6 624.7
Total, Fair Value 5,415.2 6,610.9
Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 1.4 5.9
12 months or less, Fair Value 632.1 476.0
Greater than 12 months, Gross Unrealized Losses 389.5 609.9
Greater than 12 months, Fair Value 4,663.4 6,034.0
Total, Gross Unrealized Losses 390.9 615.8
Total, Fair Value 5,295.5 6,510.0
Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.9 2.5
12 months or less, Fair Value 61.7 33.8
Greater than 12 months, Gross Unrealized Losses 4.8 6.4
Greater than 12 months, Fair Value 58.0 67.1
Total, Gross Unrealized Losses 5.7 8.9
Total, Fair Value 119.7 100.9
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 0.3 0.6
12 months or less, Fair Value 106.0 62.2
Greater than 12 months, Gross Unrealized Losses 43.7 51.1
Greater than 12 months, Fair Value 330.2 331.0
Total, Gross Unrealized Losses 44.0 51.7
Total, Fair Value 436.2 393.2
Foreign Governments [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Greater than 12 months, Fair Value   2.0
Total, Fair Value   2.0
Municipals [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.2 2.5
12 months or less, Fair Value 39.8 72.5
Greater than 12 months, Gross Unrealized Losses 89.8 127.1
Greater than 12 months, Fair Value 848.0 935.2
Total, Gross Unrealized Losses 90.0 129.6
Total, Fair Value 887.8 1,007.7
Corporates [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.6 1.3
12 months or less, Fair Value 347.4 159.9
Greater than 12 months, Gross Unrealized Losses 112.6 214.9
Greater than 12 months, Fair Value 2,005.6 2,870.2
Total, Gross Unrealized Losses 113.2 216.2
Total, Fair Value 2,353.0 3,030.1
Corporates [Member] | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.9 2.5
12 months or less, Fair Value 61.7 33.8
Greater than 12 months, Gross Unrealized Losses 4.8 6.4
Greater than 12 months, Fair Value 57.1 66.2
Total, Gross Unrealized Losses 5.7 8.9
Total, Fair Value 118.8 100.0
Residential Mortgage-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.1 1.2
12 months or less, Fair Value 78.1 139.3
Greater than 12 months, Gross Unrealized Losses 105.0 129.0
Greater than 12 months, Fair Value 808.1 865.4
Total, Gross Unrealized Losses 105.1 130.2
Total, Fair Value 886.2 1,004.7
Commercial Mortgage-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses   0.2
12 months or less, Fair Value   14.0
Greater than 12 months, Gross Unrealized Losses 36.4 72.0
Greater than 12 months, Fair Value 545.4 770.0
Total, Gross Unrealized Losses 36.4 72.2
Total, Fair Value 545.4 784.0
Commercial Mortgage-Backed [Member] | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Greater than 12 months, Fair Value 0.9 0.9
Total, Fair Value 0.9 0.9
Other Asset-Backed [Member] | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
12 months or less, Gross Unrealized Losses 0.2 0.1
12 months or less, Fair Value 60.8 28.1
Greater than 12 months, Gross Unrealized Losses 2.0 15.8
Greater than 12 months, Fair Value 126.1 260.2
Total, Gross Unrealized Losses 2.2 15.9
Total, Fair Value $ 186.9 $ 288.3