Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 7,198.0	$ 6,945.6
Amortized Cost, Net of Allowance for Credit Losses	7,198.0
Fair Value	7,555.8	7,454.4
U.S. Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	381.5	376.4
Amortized Cost, Net of Allowance for Credit Losses	381.5	376.4
Gross Unrealized Gains	12.1	17.6
Gross Unrealized Loss	6.7	2.0
Fair Value	386.9	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	1,053.9	1,042.1
Amortized Cost, Net of Allowance for Credit Losses	1,053.9	1,042.1
Gross Unrealized Gains	45.3	61.7
Gross Unrealized Loss	2.0	0.1
Fair Value	1,097.2	1,103.7
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,965.5	3,770.0
Allowance for Credit Losses	(0.1)	(0.1)
Amortized Cost, Net of Allowance for Credit Losses	3,965.4	3,769.9
Gross Unrealized Gains	256.2	341.8
Gross Unrealized Loss	8.2	0.4
Fair Value	4,213.4	4,111.3
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	980.0	978.2
Amortized Cost, Net of Allowance for Credit Losses	980.0	978.2
Gross Unrealized Gains	22.0	33.1
Gross Unrealized Loss	4.2	0.6
Fair Value	997.8	1,010.7
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	739.3	705.4
Amortized Cost, Net of Allowance for Credit Losses	739.3	705.4
Gross Unrealized Gains	41.8	54.8
Gross Unrealized Loss	1.5	0.2
Fair Value	779.6	760.0
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	75.7	71.4
Amortized Cost, Net of Allowance for Credit Losses	75.7	71.4
Gross Unrealized Gains	2.6	2.6
Fair Value	78.3	74.0
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,198.1	6,945.7
Allowance for Credit Losses	(0.1)	(0.1)
Amortized Cost, Net of Allowance for Credit Losses	7,198.0	6,945.6
Gross Unrealized Gains	380.4	512.1
Gross Unrealized Loss	22.6	3.3
Fair Value	$ 7,555.8	$ 7,454.4