Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 95.4	$ 407.8
12 months or less, Fair Value	2,888.6	5,414.1
Greater than 12 months, Gross Unrealized Losses	611.7	409.9
Greater than 12 months, Fair Value	4,075.3	1,681.5
Total, Gross Unrealized Losses	707.1	817.7
Total, Fair Value	6,963.9	7,095.6
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	91.1	394.3
12 months or less, Fair Value	2,768.3	5,160.9
Greater than 12 months, Gross Unrealized Losses	602.5	406.1
Greater than 12 months, Fair Value	4,006.8	1,664.2
Total, Gross Unrealized Losses	693.6	800.4
Total, Fair Value	6,775.1	6,825.1
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	3.8	15.8
12 months or less, Fair Value	113.5	237.3
Greater than 12 months, Gross Unrealized Losses	48.1	43.1
Greater than 12 months, Fair Value	261.5	160.7
Total, Gross Unrealized Losses	51.9	58.9
Total, Fair Value	375.0	398.0
Foreign Government [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	2.0	1.9
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	12.8	64.9
12 months or less, Fair Value	288.3	595.5
Greater than 12 months, Gross Unrealized Losses	121.6	99.3
Greater than 12 months, Fair Value	718.1	386.6
Total, Gross Unrealized Losses	134.4	164.2
Total, Fair Value	1,006.4	982.1
Corporate [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	44.3	202.2
12 months or less, Fair Value	1,480.5	2,786.2
Greater than 12 months, Gross Unrealized Losses	230.9	118.2
Greater than 12 months, Fair Value	1,710.4	489.8
Total, Gross Unrealized Losses	275.2	320.4
Total, Fair Value	3,190.9	3,276.0
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	4.3	13.5
12 months or less, Fair Value	120.3	253.2
Greater than 12 months, Gross Unrealized Losses	9.2	3.8
Greater than 12 months, Fair Value	68.5	17.3
Total, Gross Unrealized Losses	13.5	17.3
Total, Fair Value	188.8	270.5
Residential Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	13.8	51.6
12 months or less, Fair Value	393.0	642.0
Greater than 12 months, Gross Unrealized Losses	112.4	90.5
Greater than 12 months, Fair Value	640.0	387.6
Total, Gross Unrealized Losses	126.2	142.1
Total, Fair Value	1,033.0	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	12.0	48.7
12 months or less, Fair Value	284.5	663.2
Greater than 12 months, Gross Unrealized Losses	71.4	39.0
Greater than 12 months, Fair Value	557.9	164.4
Total, Gross Unrealized Losses	83.4	87.7
Total, Fair Value	842.4	827.6
Other Asset-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	4.4	11.0
12 months or less, Fair Value	206.5	234.8
Greater than 12 months, Gross Unrealized Losses	18.1	16.0
Greater than 12 months, Fair Value	118.9	75.1
Total, Gross Unrealized Losses	22.5	27.0
Total, Fair Value	$ 325.4	$ 309.9