Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 630.1	$ 42.4
12 months or less, Fair Value	6,050.1	2,223.2
Greater than 12 months, Gross Unrealized Losses	265.9	5.6
Greater than 12 months, Fair Value	936.7	111.3
Total, Gross Unrealized Losses	896.0	48.0
Total, Fair Value	6,986.8	2,334.5
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	602.1	40.8
12 months or less, Fair Value	5,814.3	2,127.4
Greater than 12 months, Gross Unrealized Losses	259.3	5.6
Greater than 12 months, Fair Value	913.0	111.3
Total, Gross Unrealized Losses	861.4	46.4
Total, Fair Value	6,727.3	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	21.5	2.3
12 months or less, Fair Value	243.2	98.0
Greater than 12 months, Gross Unrealized Losses	40.1	5.1
Greater than 12 months, Fair Value	129.7	101.3
Total, Gross Unrealized Losses	61.6	7.4
Total, Fair Value	372.9	199.3
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	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	116.6	7.9
12 months or less, Fair Value	806.3	480.8
Greater than 12 months, Gross Unrealized Losses	46.7	0.1
Greater than 12 months, Fair Value	170.9	2.5
Total, Gross Unrealized Losses	163.3	8.0
Total, Fair Value	977.2	483.3
Corporate [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	285.9	13.6
12 months or less, Fair Value	2,959.8	599.6
Greater than 12 months, Gross Unrealized Losses	85.1	0.4
Greater than 12 months, Fair Value	277.0	7.5
Total, Gross Unrealized Losses	371.0	14.0
Total, Fair Value	3,236.8	607.1
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	28.0	1.6
12 months or less, Fair Value	235.8	95.8
Greater than 12 months, Gross Unrealized Losses	6.6
Greater than 12 months, Fair Value	23.7
Total, Gross Unrealized Losses	34.6	1.6
Total, Fair Value	259.5	95.8
Residential Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	93.6	11.0
12 months or less, Fair Value	796.7	653.5
Greater than 12 months, Gross Unrealized Losses	54.4
Greater than 12 months, Fair Value	213.7
Total, Gross Unrealized Losses	148.0	11.0
Total, Fair Value	1,010.4	653.5
Commercial Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	62.6	4.6
12 months or less, Fair Value	716.9	204.0
Greater than 12 months, Gross Unrealized Losses	28.4
Greater than 12 months, Fair Value	105.0
Total, Gross Unrealized Losses	91.0	4.6
Total, Fair Value	821.9	204.0
Asset-backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	21.8	1.4
12 months or less, Fair Value	289.5	91.5
Greater than 12 months, Gross Unrealized Losses	4.6
Greater than 12 months, Fair Value	16.7
Total, Gross Unrealized Losses	26.4	1.4
Total, Fair Value	$ 306.2	$ 91.5