Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 22.6	$ 3.3
12 months or less, Fair Value	1,078.7	345.6
Greater than 12 months, Fair Value	0.5	0.7
Total, Gross Unrealized Losses	22.6	3.3
Total, Fair Value	1,079.2	346.3
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	21.6	3.3
12 months or less, Fair Value	1,036.7	341.0
Greater than 12 months, Fair Value	0.5	0.7
Total, Gross Unrealized Losses	21.6	3.3
Total, Fair Value	1,037.2	341.7
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	6.7	2.0
12 months or less, Fair Value	185.4	140.7
Total, Gross Unrealized Losses	6.7	2.0
Total, Fair Value	185.4	140.7
Municipal [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.0	0.1
12 months or less, Fair Value	176.1	19.3
Greater than 12 months, Fair Value	0.5	0.7
Total, Gross Unrealized Losses	2.0	0.1
Total, Fair Value	176.6	20.0
Corporate [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	7.2	0.4
12 months or less, Fair Value	298.4	53.4
Total, Gross Unrealized Losses	7.2	0.4
Total, Fair Value	298.4	53.4
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.0
12 months or less, Fair Value	42.0	4.6
Total, Gross Unrealized Losses	1.0
Total, Fair Value	42.0	4.6
Residential Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	4.2	0.6
12 months or less, Fair Value	304.5	94.4
Total, Gross Unrealized Losses	4.2	0.6
Total, Fair Value	304.5	94.4
Commercial Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.5	0.2
12 months or less, Fair Value	72.3	33.2
Total, Gross Unrealized Losses	1.5	0.2
Total, Fair Value	$ 72.3	$ 33.2