Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 33.5	$ 3.3
12 months or less, Fair Value	1,757.3	345.6
Greater than 12 months, Gross Unrealized Losses	1.7
Greater than 12 months, Fair Value	23.9	0.7
Total, Gross Unrealized Losses	35.2	3.3
Total, Fair Value	1,781.2	346.3
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	32.7	3.3
12 months or less, Fair Value	1,708.1	341.0
Greater than 12 months, Gross Unrealized Losses	1.7
Greater than 12 months, Fair Value	23.9	0.7
Total, Gross Unrealized Losses	34.4	3.3
Total, Fair Value	1,732.0	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	5.7	2.0
12 months or less, Fair Value	148.9	140.7
Greater than 12 months, Gross Unrealized Losses	1.6
Greater than 12 months, Fair Value	21.5
Total, Gross Unrealized Losses	7.3	2.0
Total, Fair Value	170.4	140.7
Municipal [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	5.3	0.1
12 months or less, Fair Value	372.6	19.3
Greater than 12 months, Fair Value	0.5	0.7
Total, Gross Unrealized Losses	5.3	0.1
Total, Fair Value	373.1	20.0
Corporate [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	10.5	0.4
12 months or less, Fair Value	485.8	53.4
Greater than 12 months, Gross Unrealized Losses	0.1
Greater than 12 months, Fair Value	1.9
Total, Gross Unrealized Losses	10.6	0.4
Total, Fair Value	487.7	53.4
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.8
12 months or less, Fair Value	49.2	4.6
Total, Gross Unrealized Losses	0.8
Total, Fair Value	49.2	4.6
Residential Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	7.7	0.6
12 months or less, Fair Value	526.5	94.4
Total, Gross Unrealized Losses	7.7	0.6
Total, Fair Value	526.5	94.4
Commercial Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.2	0.2
12 months or less, Fair Value	140.8	33.2
Total, Gross Unrealized Losses	3.2	0.2
Total, Fair Value	140.8	$ 33.2
Asset-backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.3
12 months or less, Fair Value	33.5
Total, Gross Unrealized Losses	0.3
Total, Fair Value	$ 33.5