Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Net of Allowance for Credit Losses	$ 7,831.7	$ 7,514.8
Fair Value	7,244.7	7,723.9
U.S. Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	414.2	394.3
Amortized Cost, Net of Allowance for Credit Losses	414.2	394.3
Gross Unrealized Gains	0.6	9.3
Gross Unrealized Loss	39.0	7.4
Fair Value	375.8	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
Fair Value	2.2	2.6
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,208.8	1,176.2
Amortized Cost, Net of Allowance for Credit Losses	1,208.8	1,176.2
Gross Unrealized Gains	3.6	32.6
Gross Unrealized Loss	111.6	8.0
Fair Value	1,100.8	1,200.8
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,977.1	3,931.5
Allowance for Credit Losses	(2.2)	(0.3)
Amortized Cost, Net of Allowance for Credit Losses	3,974.9	3,931.2
Gross Unrealized Gains	5.1	174.5
Gross Unrealized Loss	281.7	15.6
Fair Value	3,698.3	4,090.1
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,074.0	1,068.2
Amortized Cost, Net of Allowance for Credit Losses	1,074.0	1,068.2
Gross Unrealized Gains	0.3	12.4
Gross Unrealized Loss	88.9	11.0
Fair Value	985.4	1,069.6
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	881.6	802.4
Amortized Cost, Net of Allowance for Credit Losses	881.6	802.4
Gross Unrealized Gains		26.6
Gross Unrealized Loss	58.4	4.6
Fair Value	823.2	824.4
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	276.0	140.3
Amortized Cost, Net of Allowance for Credit Losses	276.0	140.3
Gross Unrealized Gains	0.1	1.3
Gross Unrealized Loss	17.1	1.4
Fair Value	259.0	140.2
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,833.9	7,515.1
Allowance for Credit Losses	(2.2)	(0.3)
Amortized Cost, Net of Allowance for Credit Losses	7,831.7	7,514.8
Gross Unrealized Gains	9.7	257.1
Gross Unrealized Loss	596.7	48.0
Fair Value	$ 7,244.7	$ 7,723.9