Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 521.0	$ 42.4
12 months or less, Fair Value	6,171.0	2,223.2
Greater than 12 months, Gross Unrealized Losses	75.7	5.6
Greater than 12 months, Fair Value	334.7	111.3
Total, Gross Unrealized Losses	596.7	48.0
Total, Fair Value	6,505.7	2,334.5
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	479.7	40.8
12 months or less, Fair Value	5,847.9	2,127.4
Greater than 12 months, Gross Unrealized Losses	73.1	5.6
Greater than 12 months, Fair Value	326.2	111.3
Total, Gross Unrealized Losses	552.8	46.4
Total, Fair Value	6,174.1	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	14.4	2.3
12 months or less, Fair Value	232.3	98.0
Greater than 12 months, Gross Unrealized Losses	24.6	5.1
Greater than 12 months, Fair Value	116.9	101.3
Total, Gross Unrealized Losses	39.0	7.4
Total, Fair Value	349.2	199.3
Foreign Government [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Fair Value	2.0
Total, Fair Value	2.0
Municipal [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	109.8	7.9
12 months or less, Fair Value	931.7	480.8
Greater than 12 months, Gross Unrealized Losses	1.8	0.1
Greater than 12 months, Fair Value	8.2	2.5
Total, Gross Unrealized Losses	111.6	8.0
Total, Fair Value	939.9	483.3
Corporate [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	210.2	13.6
12 months or less, Fair Value	2,786.1	599.6
Greater than 12 months, Gross Unrealized Losses	27.6	0.4
Greater than 12 months, Fair Value	110.9	7.5
Total, Gross Unrealized Losses	237.8	14.0
Total, Fair Value	2,897.0	607.1
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	41.3	1.6
12 months or less, Fair Value	323.1	95.8
Greater than 12 months, Gross Unrealized Losses	2.6
Greater than 12 months, Fair Value	8.5
Total, Gross Unrealized Losses	43.9	1.6
Total, Fair Value	331.6	95.8
Residential Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	77.3	11.0
12 months or less, Fair Value	890.1	653.5
Greater than 12 months, Gross Unrealized Losses	11.6
Greater than 12 months, Fair Value	51.3
Total, Gross Unrealized Losses	88.9	11.0
Total, Fair Value	941.4	653.5
Commercial Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	50.9	4.6
12 months or less, Fair Value	777.4	204.0
Greater than 12 months, Gross Unrealized Losses	7.5
Greater than 12 months, Fair Value	38.9
Total, Gross Unrealized Losses	58.4	4.6
Total, Fair Value	816.3	204.0
Asset-backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	17.1	1.4
12 months or less, Fair Value	228.3	91.5
Total, Gross Unrealized Losses	17.1	1.4
Total, Fair Value	$ 228.3	$ 91.5