Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
Allowance for Credit Losses	$ (7.2)	$ (3.2)	$ (7.1)	$ (7.1)
Amortized Cost, Net of Allowance for Credit Losses	8,373.2	8,294.5
Fair Value	7,678.2	7,481.8
U.S. Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	484.0	478.9
Amortized Cost, Net of Allowance for Credit Losses	484.0	478.9
Gross Unrealized Gains	0.8	0.2
Gross Unrealized Loss	51.9	58.9
Fair Value	432.9	420.2
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2.2	2.3
Amortized Cost, Net of Allowance for Credit Losses	2.2	2.3
Gross Unrealized Loss		0.1
Fair Value	2.2	2.2
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,258.0	1,239.5
Amortized Cost, Net of Allowance for Credit Losses	1,258.0	1,239.5
Gross Unrealized Gains	2.2	0.8
Gross Unrealized Loss	134.4	164.2
Fair Value	1,125.8	1,076.1
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,063.3	4,066.4
Allowance for Credit Losses	(2.4)	(2.1)
Amortized Cost, Net of Allowance for Credit Losses	4,060.9	4,064.3
Gross Unrealized Gains	7.7	3.3
Gross Unrealized Loss	288.7	337.7
Fair Value	3,779.9	3,729.9
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,278.0	1,215.3
Amortized Cost, Net of Allowance for Credit Losses	1,278.0	1,215.3
Gross Unrealized Gains	1.3	0.6
Gross Unrealized Loss	126.2	142.1
Fair Value	1,153.1	1,073.8
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	925.9	924.1
Amortized Cost, Net of Allowance for Credit Losses	925.9	924.1
Gross Unrealized Loss	83.4	87.7
Fair Value	842.5	836.4
Other Asset-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	364.2	370.1
Amortized Cost, Net of Allowance for Credit Losses	364.2	370.1
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Loss	22.5	27.0
Fair Value	341.8	343.2
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,375.6	8,296.6
Allowance for Credit Losses	(2.4)	(2.1)
Amortized Cost, Net of Allowance for Credit Losses	8,373.2	8,294.5
Gross Unrealized Gains	12.1	5.0
Gross Unrealized Loss	707.1	817.7
Fair Value	$ 7,678.2	$ 7,481.8