Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 7,400.4	$ 6,945.6
Amortized Cost, Net of Allowance for Credit Losses	7,400.4
Fair Value	7,699.2	7,454.4
U.S. Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	355.2	376.4
Amortized Cost, Net of Allowance for Credit Losses	355.2	376.4
Gross Unrealized Gains	10.8	17.6
Gross Unrealized Loss	7.3	2.0
Fair Value	358.7	392.0
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	0.5
Fair Value	2.6	2.7
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,104.7	1,042.1
Amortized Cost, Net of Allowance for Credit Losses	1,104.7	1,042.1
Gross Unrealized Gains	39.5	61.7
Gross Unrealized Loss	5.3	0.1
Fair Value	1,138.9	1,103.7
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,983.8	3,770.0
Allowance for Credit Losses	(0.2)	(0.1)
Amortized Cost, Net of Allowance for Credit Losses	3,983.6	3,769.9
Gross Unrealized Gains	226.6	341.8
Gross Unrealized Loss	11.4	0.4
Fair Value	4,198.8	4,111.3
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,069.1	978.2
Amortized Cost, Net of Allowance for Credit Losses	1,069.1	978.2
Gross Unrealized Gains	18.2	33.1
Gross Unrealized Loss	7.7	0.6
Fair Value	1,079.6	1,010.7
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	784.0	705.4
Amortized Cost, Net of Allowance for Credit Losses	784.0	705.4
Gross Unrealized Gains	36.3	54.8
Gross Unrealized Loss	3.2	0.2
Fair Value	817.1	760.0
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	101.6	71.4
Amortized Cost, Net of Allowance for Credit Losses	101.6	71.4
Gross Unrealized Gains	2.2	2.6
Gross Unrealized Loss	0.3
Fair Value	103.5	74.0
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,400.6	6,945.7
Allowance for Credit Losses	(0.2)	(0.1)
Amortized Cost, Net of Allowance for Credit Losses	7,400.4	6,945.6
Gross Unrealized Gains	334.0	512.1
Gross Unrealized Loss	35.2	3.3
Fair Value	$ 7,699.2	$ 7,454.4