Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Schedule of Available-for-sale Securities [Line Items]
Allowance for Credit Losses	$ (9.7)	$ (10.0)	$ (7.2)	$ (3.2)
Amortized Cost, Net of Allowance for Credit Losses	8,645.4	8,573.9
Fair Value	8,015.4	7,985.3
U.S. Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	520.0	512.9
Amortized Cost, Net of Allowance for Credit Losses	520.0	512.9
Gross Unrealized Gains	0.5	1.4
Gross Unrealized Loss	58.8	51.7
Fair Value	461.7	462.6
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	3.5	2.2
Amortized Cost, Net of Allowance for Credit Losses	3.5	2.2
Fair Value	3.5	2.2
Municipals [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	1,300.7	1,309.3
Amortized Cost, Net of Allowance for Credit Losses	1,300.7	1,309.3
Gross Unrealized Gains	3.8	5.5
Gross Unrealized Loss	131.5	129.6
Fair Value	1,173.0	1,185.2
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	4,109.2	4,053.8
Allowance for Credit Losses	(1.6)	(1.9)
Amortized Cost, Net of Allowance for Credit Losses	4,107.6	4,051.9
Gross Unrealized Gains	16.6	22.9
Gross Unrealized Loss	235.8	225.1
Fair Value	3,888.4	3,849.7
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	1,469.1	1,435.6
Amortized Cost, Net of Allowance for Credit Losses	1,469.1	1,435.6
Gross Unrealized Gains	2.7	5.5
Gross Unrealized Loss	145.7	130.2
Fair Value	1,326.1	1,310.9
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	873.0	890.3
Amortized Cost, Net of Allowance for Credit Losses	873.0	890.3
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Loss	67.5	72.2
Fair Value	805.6	818.2
Other Asset-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	371.5	371.7
Amortized Cost, Net of Allowance for Credit Losses	371.5	371.7
Gross Unrealized Gains	0.5	0.7
Gross Unrealized Loss	14.9	15.9
Fair Value	357.1	356.5
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	8,647.0	8,575.8
Allowance for Credit Losses	(1.6)	(1.9)
Amortized Cost, Net of Allowance for Credit Losses	8,645.4	8,573.9
Gross Unrealized Gains	24.2	36.1
Gross Unrealized Loss	654.2	624.7
Fair Value	$ 8,015.4	$ 7,985.3