Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 262.1	$ 42.4
12 months or less, Fair Value	4,397.1	2,223.2
Greater than 12 months, Gross Unrealized Losses	50.1	5.6
Greater than 12 months, Fair Value	373.2	111.3
Total, Gross Unrealized Losses	312.2	48.0
Total, Fair Value	4,770.3	2,334.5
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	248.6	40.8
12 months or less, Fair Value	4,140.0	2,127.4
Greater than 12 months, Gross Unrealized Losses	48.9	5.6
Greater than 12 months, Fair Value	363.8	111.3
Total, Gross Unrealized Losses	297.5	46.4
Total, Fair Value	4,503.8	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	7.5	2.3
12 months or less, Fair Value	132.1	98.0
Greater than 12 months, Gross Unrealized Losses	16.9	5.1
Greater than 12 months, Fair Value	124.6	101.3
Total, Gross Unrealized Losses	24.4	7.4
Total, Fair Value	256.7	199.3
Municipal [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	61.1	7.9
12 months or less, Fair Value	775.3	480.8
Greater than 12 months, Gross Unrealized Losses	1.3	0.1
Greater than 12 months, Fair Value	8.9	2.5
Total, Gross Unrealized Losses	62.4	8.0
Total, Fair Value	784.2	483.3
Corporate [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	91.4	13.6
12 months or less, Fair Value	1,706.1	599.6
Greater than 12 months, Gross Unrealized Losses	17.0	0.4
Greater than 12 months, Fair Value	115.5	7.5
Total, Gross Unrealized Losses	108.4	14.0
Total, Fair Value	1,821.6	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	257.1	95.8
Greater than 12 months, Gross Unrealized Losses	1.2
Greater than 12 months, Fair Value	9.4
Total, Gross Unrealized Losses	14.7	1.6
Total, Fair Value	266.5	95.8
Residential Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	54.0	11.0
12 months or less, Fair Value	822.2	653.5
Greater than 12 months, Gross Unrealized Losses	8.6
Greater than 12 months, Fair Value	73.5
Total, Gross Unrealized Losses	62.6	11.0
Total, Fair Value	895.7	653.5
Commercial Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	24.5	4.6
12 months or less, Fair Value	581.7	204.0
Greater than 12 months, Gross Unrealized Losses	5.1
Greater than 12 months, Fair Value	41.3
Total, Gross Unrealized Losses	29.6	4.6
Total, Fair Value	623.0	204.0
Asset-backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	10.1	1.4
12 months or less, Fair Value	122.6	91.5
Total, Gross Unrealized Losses	10.1	1.4
Total, Fair Value	$ 122.6	$ 91.5