Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Net of Allowance for Credit Losses	$ 8,294.5	$ 7,514.8
Fair Value	7,481.8	7,723.9
U.S. Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	478.9	394.3
Amortized Cost, Net of Allowance for Credit Losses	478.9	394.3
Gross Unrealized Gains	0.2	9.3
Gross Unrealized Loss	58.9	7.4
Fair Value	420.2	396.2
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	2.3	2.2
Amortized Cost, Net of Allowance for Credit Losses	2.3	2.2
Gross Unrealized Gains		0.4
Gross Unrealized Loss	0.1
Fair Value	2.2	2.6
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	1,239.5	1,176.2
Amortized Cost, Net of Allowance for Credit Losses	1,239.5	1,176.2
Gross Unrealized Gains	0.8	32.6
Gross Unrealized Loss	164.2	8.0
Fair Value	1,076.1	1,200.8
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	4,066.4	3,931.5
Allowance for Credit Losses	(2.1)	(0.3)
Amortized Cost, Net of Allowance for Credit Losses	4,064.3	3,931.2
Gross Unrealized Gains	3.3	174.5
Gross Unrealized Loss	337.7	15.6
Fair Value	3,729.9	4,090.1
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	1,215.3	1,068.2
Amortized Cost, Net of Allowance for Credit Losses	1,215.3	1,068.2
Gross Unrealized Gains	0.6	12.4
Gross Unrealized Loss	142.1	11.0
Fair Value	1,073.8	1,069.6
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	924.1	802.4
Amortized Cost, Net of Allowance for Credit Losses	924.1	802.4
Gross Unrealized Gains		26.6
Gross Unrealized Loss	87.7	4.6
Fair Value	836.4	824.4
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	370.1	140.3
Amortized Cost, Net of Allowance for Credit Losses	370.1	140.3
Gross Unrealized Gains	0.1	1.3
Gross Unrealized Loss	27.0	1.4
Fair Value	343.2	140.2
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	8,296.6	7,515.1
Allowance for Credit Losses	(2.1)	(0.3)
Amortized Cost, Net of Allowance for Credit Losses	8,294.5	7,514.8
Gross Unrealized Gains	5.0	257.1
Gross Unrealized Loss	817.7	48.0
Fair Value	$ 7,481.8	$ 7,723.9