Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 23.8	$ 54.6
12 months or less, Fair Value	1,931.1	2,860.0
Greater than 12 months, Gross Unrealized Losses	392.6	476.5
Greater than 12 months, Fair Value	3,374.3	3,806.7
Total, Gross Unrealized Losses	416.4	531.1
Total, Fair Value	5,305.4	6,666.7
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	20.4	53.3
12 months or less, Fair Value	1,781.8	2,789.1
Greater than 12 months, Gross Unrealized Losses	386.4	471.2
Greater than 12 months, Fair Value	3,324.0	3,756.1
Total, Gross Unrealized Losses	406.8	524.5
Total, Fair Value	5,105.8	6,545.2
Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.4	1.3
12 months or less, Fair Value	149.3	70.9
Greater than 12 months, Gross Unrealized Losses	6.2	5.3
Greater than 12 months, Fair Value	50.3	50.6
Total, Gross Unrealized Losses	9.6	6.6
Total, Fair Value	199.6	121.5
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	3.4	7.8
12 months or less, Fair Value	129.1	194.4
Greater than 12 months, Gross Unrealized Losses	44.3	51.0
Greater than 12 months, Fair Value	244.2	240.8
Total, Gross Unrealized Losses	47.7	58.8
Total, Fair Value	373.3	435.2
Foreign Governments [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Fair Value	0.4	0.1
Total, Fair Value	0.4	0.1
Municipals [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.5	3.0
12 months or less, Fair Value	108.0	132.5
Greater than 12 months, Gross Unrealized Losses	89.0	106.2
Greater than 12 months, Fair Value	658.3	680.9
Total, Gross Unrealized Losses	90.5	109.2
Total, Fair Value	766.3	813.4
Corporates [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	8.6	22.8
12 months or less, Fair Value	773.4	1,360.3
Greater than 12 months, Gross Unrealized Losses	99.1	132.0
Greater than 12 months, Fair Value	1,240.9	1,523.3
Total, Gross Unrealized Losses	107.7	154.8
Total, Fair Value	2,014.3	2,883.6
Corporates [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.4	1.3
12 months or less, Fair Value	148.9	70.8
Greater than 12 months, Gross Unrealized Losses	5.2	5.3
Greater than 12 months, Fair Value	45.2	49.8
Total, Gross Unrealized Losses	8.6	6.6
Total, Fair Value	194.1	120.6
Residential Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	4.5	16.5
12 months or less, Fair Value	476.3	896.8
Greater than 12 months, Gross Unrealized Losses	122.2	139.8
Greater than 12 months, Fair Value	742.1	747.2
Total, Gross Unrealized Losses	126.7	156.3
Total, Fair Value	1,218.4	1,644.0
Commercial Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.3	0.7
12 months or less, Fair Value	26.8	23.7
Greater than 12 months, Gross Unrealized Losses	31.2	41.4
Greater than 12 months, Fair Value	364.4	485.4
Total, Gross Unrealized Losses	31.5	42.1
Total, Fair Value	391.2	509.1
Commercial Mortgage-Backed [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Greater than 12 months, Gross Unrealized Losses	1.0
Greater than 12 months, Fair Value	5.1	0.8
Total, Gross Unrealized Losses	1.0
Total, Fair Value	5.1	0.8
Other Asset-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.1	2.5
12 months or less, Fair Value	268.2	181.4
Greater than 12 months, Gross Unrealized Losses	0.6	0.8
Greater than 12 months, Fair Value	74.1	78.5
Total, Gross Unrealized Losses	2.7	3.3
Total, Fair Value	$ 342.3	$ 259.9