Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 3.3	$ 6.1
12 months or less, Fair Value	345.6	368.9
Greater than 12 months, Gross Unrealized Losses		1.5
Greater than 12 months, Fair Value	0.7	65.6
Total, Gross Unrealized Losses	3.3	7.6
Total, Fair Value	346.3	434.5
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.3	3.9
12 months or less, Fair Value	341.0	341.8
Greater than 12 months, Gross Unrealized Losses		0.6
Greater than 12 months, Fair Value	0.7	56.6
Total, Gross Unrealized Losses	3.3	4.5
Total, Fair Value	341.7	398.4
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	2.0	1.3
12 months or less, Fair Value	140.7	73.0
Greater than 12 months, Fair Value		9.3
Total, Gross Unrealized Losses	2.0	1.3
Total, Fair Value	140.7	82.3
Municipal [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.1	1.1
12 months or less, Fair Value	19.3	72.5
Greater than 12 months, Gross Unrealized Losses		0.1
Greater than 12 months, Fair Value	0.7	5.6
Total, Gross Unrealized Losses	0.1	1.2
Total, Fair Value	20.0	78.1
Corporate [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.4	0.7
12 months or less, Fair Value	53.4	86.5
Greater than 12 months, Gross Unrealized Losses		0.1
Greater than 12 months, Fair Value		4.7
Total, Gross Unrealized Losses	0.4	0.8
Total, Fair Value	53.4	91.2
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		2.2
12 months or less, Fair Value	4.6	27.1
Greater than 12 months, Gross Unrealized Losses		0.9
Greater than 12 months, Fair Value		9.0
Total, Gross Unrealized Losses		3.1
Total, Fair Value	4.6	36.1
Residential Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.6	0.7
12 months or less, Fair Value	94.4	69.2
Greater than 12 months, Gross Unrealized Losses		0.4
Greater than 12 months, Fair Value		34.4
Total, Gross Unrealized Losses	0.6	1.1
Total, Fair Value	94.4	103.6
Commercial Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.2	0.1
12 months or less, Fair Value	33.2	40.6
Greater than 12 months, Fair Value		0.9
Total, Gross Unrealized Losses	0.2	0.1
Total, Fair Value	$ 33.2	41.5
Asset-backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Greater than 12 months, Fair Value		1.7
Total, Fair Value		$ 1.7