Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Net of Allowance for Credit Losses	$ 8,093.9	$ 7,514.8
Fair Value	7,200.9	7,723.9
U.S. Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	441.3	394.3
Amortized Cost, Net of Allowance for Credit Losses	441.3	394.3
Gross Unrealized Gains	0.2	9.3
Gross Unrealized Loss	61.6	7.4
Fair Value	379.9	396.2
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2.2	2.2
Amortized Cost, Net of Allowance for Credit Losses	2.2	2.2
Gross Unrealized Gains		0.4
Gross Unrealized Loss	0.1
Fair Value	2.1	2.6
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,214.3	1,176.2
Amortized Cost, Net of Allowance for Credit Losses	1,214.3	1,176.2
Gross Unrealized Gains	0.8	32.6
Gross Unrealized Loss	163.3	8.0
Fair Value	1,051.8	1,200.8
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,023.5	3,931.5
Allowance for Credit Losses	(3.1)	(0.3)
Amortized Cost, Net of Allowance for Credit Losses	4,020.4	3,931.2
Gross Unrealized Gains	2.0	174.5
Gross Unrealized Loss	405.6	15.6
Fair Value	3,616.8	4,090.1
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,166.3	1,068.2
Amortized Cost, Net of Allowance for Credit Losses	1,166.3	1,068.2
Gross Unrealized Gains		12.4
Gross Unrealized Loss	148.0	11.0
Fair Value	1,018.3	1,069.6
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	916.8	802.4
Amortized Cost, Net of Allowance for Credit Losses	916.8	802.4
Gross Unrealized Gains		26.6
Gross Unrealized Loss	91.0	4.6
Fair Value	825.8	824.4
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	332.6	140.3
Amortized Cost, Net of Allowance for Credit Losses	332.6	140.3
Gross Unrealized Gains		1.3
Gross Unrealized Loss	26.4	1.4
Fair Value	306.2	140.2
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,097.0	7,515.1
Allowance for Credit Losses	(3.1)	(0.3)
Amortized Cost, Net of Allowance for Credit Losses	8,093.9	7,514.8
Gross Unrealized Gains	3.0	257.1
Gross Unrealized Loss	896.0	48.0
Fair Value	$ 7,200.9	$ 7,723.9