Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 407.8	$ 42.4
12 months or less, Fair Value	5,414.1	2,223.2
Greater than 12 months, Gross Unrealized Losses	409.9	5.6
Greater than 12 months, Fair Value	1,681.5	111.3
Total, Gross Unrealized Losses	817.7	48.0
Total, Fair Value	7,095.6	2,334.5
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	394.3	40.8
12 months or less, Fair Value	5,160.9	2,127.4
Greater than 12 months, Gross Unrealized Losses	406.1	5.6
Greater than 12 months, Fair Value	1,664.2	111.3
Total, Gross Unrealized Losses	800.4	46.4
Total, Fair Value	6,825.1	2,238.7
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	15.8	2.3
12 months or less, Fair Value	237.3	98.0
Greater than 12 months, Gross Unrealized Losses	43.1	5.1
Greater than 12 months, Fair Value	160.7	101.3
Total, Gross Unrealized Losses	58.9	7.4
Total, Fair Value	398.0	199.3
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
Total, Gross Unrealized Losses	0.1
Total, Fair Value	1.9
Municipal [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	64.9	7.9
12 months or less, Fair Value	595.5	480.8
Greater than 12 months, Gross Unrealized Losses	99.3	0.1
Greater than 12 months, Fair Value	386.6	2.5
Total, Gross Unrealized Losses	164.2	8.0
Total, Fair Value	982.1	483.3
Corporate [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	202.2	13.6
12 months or less, Fair Value	2,786.2	599.6
Greater than 12 months, Gross Unrealized Losses	118.2	0.4
Greater than 12 months, Fair Value	489.8	7.5
Total, Gross Unrealized Losses	320.4	14.0
Total, Fair Value	3,276.0	607.1
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	13.5	1.6
12 months or less, Fair Value	253.2	95.8
Greater than 12 months, Gross Unrealized Losses	3.8
Greater than 12 months, Fair Value	17.3
Total, Gross Unrealized Losses	17.3	1.6
Total, Fair Value	270.5	95.8
Residential Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	51.6	11.0
12 months or less, Fair Value	642.0	653.5
Greater than 12 months, Gross Unrealized Losses	90.5
Greater than 12 months, Fair Value	387.6
Total, Gross Unrealized Losses	142.1	11.0
Total, Fair Value	1,029.6	653.5
Commercial Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	48.7	4.6
12 months or less, Fair Value	663.2	204.0
Greater than 12 months, Gross Unrealized Losses	39.0
Greater than 12 months, Fair Value	164.4
Total, Gross Unrealized Losses	87.7	4.6
Total, Fair Value	827.6	204.0
Asset-backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	11.0	1.4
12 months or less, Fair Value	234.8	91.5
Greater than 12 months, Gross Unrealized Losses	16.0
Greater than 12 months, Fair Value	75.1
Total, Gross Unrealized Losses	27.0	1.4
Total, Fair Value	$ 309.9	$ 91.5