Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
Allowance for Credit Losses	$ (10.0)	$ (3.2)	$ (7.1)
Amortized Cost, Net of Allowance for Credit Losses	8,573.9	8,294.5
Fair Value	7,985.3	7,481.8
U.S. Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	512.9	478.9
Amortized Cost, Net of Allowance for Credit Losses	512.9	478.9
Gross Unrealized Gains	1.4	0.2
Gross Unrealized Loss	51.7	58.9
Fair Value	462.6	420.2
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, 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]
Fixed maturities, amortized cost	1,309.3	1,239.5
Amortized Cost, Net of Allowance for Credit Losses	1,309.3	1,239.5
Gross Unrealized Gains	5.5	0.8
Gross Unrealized Loss	129.6	164.2
Fair Value	1,185.2	1,076.1
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	4,053.8	4,066.4
Allowance for Credit Losses	(1.9)	(2.1)
Amortized Cost, Net of Allowance for Credit Losses	4,051.9	4,064.3
Gross Unrealized Gains	22.9	3.3
Gross Unrealized Loss	225.1	337.7
Fair Value	3,849.7	3,729.9
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	1,435.6	1,215.3
Amortized Cost, Net of Allowance for Credit Losses	1,435.6	1,215.3
Gross Unrealized Gains	5.5	0.6
Gross Unrealized Loss	130.2	142.1
Fair Value	1,310.9	1,073.8
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	890.3	924.1
Amortized Cost, Net of Allowance for Credit Losses	890.3	924.1
Gross Unrealized Gains	0.1
Gross Unrealized Loss	72.2	87.7
Fair Value	818.2	836.4
Other asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	371.7	370.1
Amortized Cost, Net of Allowance for Credit Losses	371.7	370.1
Gross Unrealized Gains	0.7	0.1
Gross Unrealized Loss	15.9	27.0
Fair Value	356.5	343.2
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	8,575.8	8,296.6
Allowance for Credit Losses	(1.9)	(2.1)
Amortized Cost, Net of Allowance for Credit Losses	8,573.9	8,294.5
Gross Unrealized Gains	36.1	5.0
Gross Unrealized Loss	624.7	817.7
Fair Value	$ 7,985.3	$ 7,481.8