Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Schedule of Available-for-sale Securities [Line Items]
Allowance for Credit Losses	$ (5.7)	$ (5.7)	$ (9.7)	$ (10.0)
Amortized Cost, Net of Allowance for Credit Losses	9,178.7	9,051.5
Fair Value	8,808.3	8,542.2
U.S. Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	538.8	552.6
Amortized Cost, Net of Allowance for Credit Losses	538.8	552.6
Gross Unrealized Gains	1.6	0.4
Gross Unrealized Loss	47.7	58.8
Fair Value	492.7	494.2
Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	2.3	1.8
Amortized Cost, Net of Allowance for Credit Losses	2.3	1.8
Gross Unrealized Gains	0.1	0.1
Fair Value	2.4	1.9
Municipals [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	972.4	1,001.5
Amortized Cost, Net of Allowance for Credit Losses	972.4	1,001.5
Gross Unrealized Gains	2.0	1.5
Gross Unrealized Loss	90.5	109.2
Fair Value	883.9	893.8
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	4,030.6	3,953.5
Allowance for Credit Losses	(0.5)	(0.6)
Amortized Cost, Net of Allowance for Credit Losses	4,030.1	3,952.9
Gross Unrealized Gains	25.0	14.4
Gross Unrealized Loss	116.3	161.4
Fair Value	3,938.8	3,805.9
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	2,375.7	2,277.6
Amortized Cost, Net of Allowance for Credit Losses	2,375.7	2,277.6
Gross Unrealized Gains	14.0	2.8
Gross Unrealized Loss	126.7	156.3
Fair Value	2,263.0	2,124.1
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	436.9	564.2
Amortized Cost, Net of Allowance for Credit Losses	436.9	564.2
Gross Unrealized Gains	0.2	0.1
Gross Unrealized Loss	32.5	42.1
Fair Value	404.6	522.2
Other Asset-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	822.5	700.9
Amortized Cost, Net of Allowance for Credit Losses	822.5	700.9
Gross Unrealized Gains	3.1	2.5
Gross Unrealized Loss	2.7	3.3
Fair Value	822.9	700.1
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	9,179.2	9,052.1
Allowance for Credit Losses	(0.5)	(0.6)
Amortized Cost, Net of Allowance for Credit Losses	9,178.7	9,051.5
Gross Unrealized Gains	46.0	21.8
Gross Unrealized Loss	416.4	531.1
Fair Value	$ 8,808.3	$ 8,542.2