Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 7,089,100,000	$ 6,945,600,000
Amortized Cost, Net of Allowance for Credit Losses	7,089,100,000
Fair Value	7,370,300,000	7,454,400,000
U.S. Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	360,700,000	376,400,000
Amortized Cost, Net of Allowance for Credit Losses	360,700,000	376,400,000
Gross Unrealized Gains	11,100,000	17,600,000
Gross Unrealized Loss	12,100,000	2,000,000.0
Fair Value	359,700,000	392,000,000.0
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,200,000	2,200,000
Amortized Cost, Net of Allowance for Credit Losses	2,200,000	2,200,000
Gross Unrealized Gains	400,000	500,000
Fair Value	2,600,000	2,700,000
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,046,300,000	1,042,100,000
Amortized Cost, Net of Allowance for Credit Losses	1,046,300,000	1,042,100,000
Gross Unrealized Gains	38,500,000	61,700,000
Gross Unrealized Loss	11,700,000	100,000
Fair Value	1,073,100,000	1,103,700,000
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,908,600,000	3,770,000,000.0
Allowance for Credit Losses		(100,000)
Amortized Cost, Net of Allowance for Credit Losses	3,908,600,000	3,769,900,000
Gross Unrealized Gains	227,500,000	341,800,000
Gross Unrealized Loss	20,900,000	400,000
Fair Value	4,115,200,000	4,111,300,000
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	981,500,000	978,200,000
Amortized Cost, Net of Allowance for Credit Losses	981,500,000	978,200,000
Gross Unrealized Gains	23,100,000	33,100,000
Gross Unrealized Loss	10,400,000	600,000
Fair Value	994,200,000	1,010,700,000
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	722,800,000	705,400,000
Amortized Cost, Net of Allowance for Credit Losses	722,800,000	705,400,000
Gross Unrealized Gains	36,700,000	54,800,000
Gross Unrealized Loss	3,000,000.0	200,000
Fair Value	756,500,000	760,000,000.0
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	67,000,000.0	71,400,000
Amortized Cost, Net of Allowance for Credit Losses	67,000,000.0	71,400,000
Gross Unrealized Gains	2,000,000.0	2,600,000
Fair Value	69,000,000.0	74,000,000.0
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,089,100,000	6,945,700,000
Allowance for Credit Losses	0	(100,000)
Amortized Cost, Net of Allowance for Credit Losses	7,089,100,000	6,945,600,000
Gross Unrealized Gains	339,300,000	512,100,000
Gross Unrealized Loss	58,100,000	3,300,000
Fair Value	$ 7,370,300,000	$ 7,454,400,000