Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 8.4	$ 407.8
12 months or less, Fair Value	509.8	5,414.1
Greater than 12 months, Gross Unrealized Losses	616.3	409.9
Greater than 12 months, Fair Value	6,101.1	1,681.5
Total, Gross Unrealized Losses	624.7	817.7
Total, Fair Value	6,610.9	7,095.6
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	5.9	394.3
12 months or less, Fair Value	476.0	5,160.9
Greater than 12 months, Gross Unrealized Losses	609.9	406.1
Greater than 12 months, Fair Value	6,034.0	1,664.2
Total, Gross Unrealized Losses	615.8	800.4
Total, Fair Value	6,510.0	6,825.1
Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.5
12 months or less, Fair Value	33.8
Greater than 12 months, Gross Unrealized Losses	6.4
Greater than 12 months, Fair Value	67.1
Total, Gross Unrealized Losses	8.9
Total, Fair Value	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	0.6	15.8
12 months or less, Fair Value	62.2	237.3
Greater than 12 months, Gross Unrealized Losses	51.1	43.1
Greater than 12 months, Fair Value	331.0	160.7
Total, Gross Unrealized Losses	51.7	58.9
Total, Fair Value	393.2	398.0
Foreign governments [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.1
12 months or less, Fair Value		1.9
Greater than 12 months, Fair Value	2.0
Total, Gross Unrealized Losses		0.1
Total, Fair Value	2.0	1.9
Municipal [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.5	64.9
12 months or less, Fair Value	72.5	595.5
Greater than 12 months, Gross Unrealized Losses	127.1	99.3
Greater than 12 months, Fair Value	935.2	386.6
Total, Gross Unrealized Losses	129.6	164.2
Total, Fair Value	1,007.7	982.1
Corporate [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.3	202.2
12 months or less, Fair Value	159.9	2,786.2
Greater than 12 months, Gross Unrealized Losses	214.9	118.2
Greater than 12 months, Fair Value	2,870.2	489.8
Total, Gross Unrealized Losses	216.2	320.4
Total, Fair Value	3,030.1	3,276.0
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.5	13.5
12 months or less, Fair Value	33.8	253.2
Greater than 12 months, Gross Unrealized Losses	6.4	3.8
Greater than 12 months, Fair Value	66.2	17.3
Total, Gross Unrealized Losses	8.9	17.3
Total, Fair Value	100.0	270.5
Residential Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.2	51.6
12 months or less, Fair Value	139.3	642.0
Greater than 12 months, Gross Unrealized Losses	129.0	90.5
Greater than 12 months, Fair Value	865.4	387.6
Total, Gross Unrealized Losses	130.2	142.1
Total, Fair Value	1,004.7	1,029.6
Commercial Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.2	48.7
12 months or less, Fair Value	14.0	663.2
Greater than 12 months, Gross Unrealized Losses	72.0	39.0
Greater than 12 months, Fair Value	770.0	164.4
Total, Gross Unrealized Losses	72.2	87.7
Total, Fair Value	784.0	827.6
Commercial Mortgage-Backed [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Greater than 12 months, Fair Value	0.9
Total, Fair Value	0.9
Other asset-backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.1	11.0
12 months or less, Fair Value	28.1	234.8
Greater than 12 months, Gross Unrealized Losses	15.8	16.0
Greater than 12 months, Fair Value	260.2	75.1
Total, Gross Unrealized Losses	15.9	27.0
Total, Fair Value	$ 288.3	$ 309.9