Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 58.1	$ 3.3
12 months or less, Fair Value	1,618.0	345.6
Greater than 12 months, Fair Value	1.6	0.7
Total, Gross Unrealized Losses	58.1	3.3
Total, Fair Value	1,619.6	346.3
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	56.7	3.3
12 months or less, Fair Value	1,553.4	341.0
Greater than 12 months, Fair Value	1.6	0.7
Total, Gross Unrealized Losses	56.7	3.3
Total, Fair Value	1,555.0	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	12.1	2.0
12 months or less, Fair Value	171.5	140.7
Total, Gross Unrealized Losses	12.1	2.0
Total, Fair Value	171.5	140.7
Municipal [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	11.7	0.1
12 months or less, Fair Value	372.9	19.3
Greater than 12 months, Fair Value	0.5	0.7
Total, Gross Unrealized Losses	11.7	0.1
Total, Fair Value	373.4	20.0
Corporate [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	19.5	0.4
12 months or less, Fair Value	515.1	53.4
Total, Gross Unrealized Losses	19.5	0.4
Total, Fair Value	515.1	53.4
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.4
12 months or less, Fair Value	64.6	4.6
Total, Gross Unrealized Losses	1.4
Total, Fair Value	64.6	4.6
Residential Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	10.4	0.6
12 months or less, Fair Value	405.2	94.4
Greater than 12 months, Fair Value	0.1
Total, Gross Unrealized Losses	10.4	0.6
Total, Fair Value	405.3	94.4
Commercial Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.0	0.2
12 months or less, Fair Value	84.1	33.2
Greater than 12 months, Fair Value	1.0
Total, Gross Unrealized Losses	3.0	0.2
Total, Fair Value	85.1	$ 33.2
Asset-backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Fair Value	4.6
Total, Fair Value	$ 4.6