Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($)		Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 6,945,600,000	$ 6,452,200,000
Amortized Cost, Net of Allowance for Credit Losses		6,945,600,000
Fair Value		7,454,400,000	6,687,100,000
U.S. Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		376,400,000	342,000,000.0
Amortized Cost, Net of Allowance for Credit Losses		376,400,000
Gross Unrealized Gains		17,600,000	9,100,000
Gross Unrealized Loss		2,000,000.0	1,300,000
Fair Value		392,000,000.0	349,800,000
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		2,200,000	15,700,000
Amortized Cost, Net of Allowance for Credit Losses		2,200,000
Gross Unrealized Gains		500,000	400,000
Fair Value		2,700,000	16,100,000
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,042,100,000	807,100,000
Amortized Cost, Net of Allowance for Credit Losses		1,042,100,000
Gross Unrealized Gains		61,700,000	27,600,000
Gross Unrealized Loss		100,000	1,200,000
Fair Value		1,103,700,000	833,500,000
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		3,770,000,000.0	3,653,500,000
Allowance for Credit Losses		(100,000)
Amortized Cost, Net of Allowance for Credit Losses		3,769,900,000
Gross Unrealized Gains		341,800,000	161,600,000
Gross Unrealized Loss		400,000	3,900,000
Fair Value		4,111,300,000	3,811,200,000
OTTI Unrealized Losses	[1]		3,000,000.0
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		978,200,000	905,400,000
Amortized Cost, Net of Allowance for Credit Losses		978,200,000
Gross Unrealized Gains		33,100,000	17,100,000
Gross Unrealized Loss		600,000	1,100,000
Fair Value		1,010,700,000	921,400,000
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		705,400,000	666,400,000
Amortized Cost, Net of Allowance for Credit Losses		705,400,000
Gross Unrealized Gains		54,800,000	25,600,000
Gross Unrealized Loss		200,000	100,000
Fair Value		760,000,000.0	691,900,000
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		71,400,000	62,100,000
Amortized Cost, Net of Allowance for Credit Losses		71,400,000
Gross Unrealized Gains		2,600,000	1,100,000
Fair Value		74,000,000.0	63,200,000
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		6,945,700,000	6,452,200,000
Allowance for Credit Losses		(100,000)	0
Amortized Cost, Net of Allowance for Credit Losses		6,945,600,000
Gross Unrealized Gains		512,100,000	242,500,000
Gross Unrealized Loss		3,300,000	7,600,000
Fair Value		$ 7,454,400,000	6,687,100,000
OTTI Unrealized Losses	[1]		$ 3,000,000.0

[1] Other-than-temporary impairments (“OTTI”) unrealized losses represents OTTI recognized in accumulated other comprehensive income. This amount excludes net unrealized gains on impaired securities relating to changes in the value of such securities subsequent to the impairment measurement date through December 31, 2019 of $4.2 million