Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 10.0	$ 8.4
12 months or less, Fair Value	815.0	509.8
Greater than 12 months, Gross Unrealized Losses	644.2	616.3
Greater than 12 months, Fair Value	5,991.6	6,101.1
Total, Gross Unrealized Losses	654.2	624.7
Total, Fair Value	6,806.6	6,610.9
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	8.8	5.9
12 months or less, Fair Value	781.0	476.0
Greater than 12 months, Gross Unrealized Losses	635.8	609.9
Greater than 12 months, Fair Value	5,923.7	6,034.0
Total, Gross Unrealized Losses	644.6	615.8
Total, Fair Value	6,704.7	6,510.0
Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		2.5
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	1.6	0.6
12 months or less, Fair Value	98.3	62.2
Greater than 12 months, Gross Unrealized Losses	57.2	51.1
Greater than 12 months, Fair Value	323.1	331.0
Total, Gross Unrealized Losses	58.8	51.7
Total, Fair Value	421.4	393.2
Foreign Government [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Greater than 12 months, Fair Value	1.9	2.0
Total, Fair Value	1.9	2.0
Municipals [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.1	2.5
12 months or less, Fair Value	85.3	72.5
Greater than 12 months, Gross Unrealized Losses	130.4	127.1
Greater than 12 months, Fair Value	943.8	935.2
Total, Gross Unrealized Losses	131.5	129.6
Total, Fair Value	1,029.1	1,007.7
Corporate [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.3	1.3
12 months or less, Fair Value	260.7	159.9
Greater than 12 months, Gross Unrealized Losses	223.9	214.9
Greater than 12 months, Fair Value	2,779.6	2,870.2
Total, Gross Unrealized Losses	226.2	216.2
Total, Fair Value	3,040.3	3,030.1
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.2	2.5
12 months or less, Fair Value	34.0	33.8
Greater than 12 months, Gross Unrealized Losses	8.4	6.4
Greater than 12 months, Fair Value	67.9	66.2
Total, Gross Unrealized Losses	9.6	8.9
Total, Fair Value	101.9	100.0
Residential Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.5	1.2
12 months or less, Fair Value	286.9	139.3
Greater than 12 months, Gross Unrealized Losses	142.2	129.0
Greater than 12 months, Fair Value	859.1	865.4
Total, Gross Unrealized Losses	145.7	130.2
Total, Fair Value	1,146.0	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	0.1	0.2
12 months or less, Fair Value	8.4	14.0
Greater than 12 months, Gross Unrealized Losses	67.4	72.0
Greater than 12 months, Fair Value	763.8	770.0
Total, Gross Unrealized Losses	67.5	72.2
Total, Fair Value	772.2	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
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	0.2	0.1
12 months or less, Fair Value	41.4	28.1
Greater than 12 months, Gross Unrealized Losses	14.7	15.8
Greater than 12 months, Fair Value	252.4	260.2
Total, Gross Unrealized Losses	14.9	15.9
Total, Fair Value	$ 293.8	$ 288.3