Investments (Current And Long-Term Fixed Maturity Securities, Available-For-Sale) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Investments [Line Items]
Gross Unrealized Gains	$ 199.7		$ 941.6
Fixed Maturities [Member]
Investments [Line Items]
Cost or Amortized Cost	17,642.4	$ 17,610.5
Gross Unrealized Gains	199.7	414.5
Gross Unrealized Losses, Less than 12 Months	(210.8)	(50.8)
Gross Unrealized Losses, 12 Months or Greater	(62.2)	(36.1)
Available-for-sale Securities	17,569.1	17,938.1
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	(0.3)
Fixed Maturities [Member] | United States Government Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	498.0	649.0
Gross Unrealized Gains	0.8	2.2
Gross Unrealized Losses, Less than 12 Months	(6.3)	(5.0)
Gross Unrealized Losses, 12 Months or Greater	(0.8)	(0.7)
Available-for-sale Securities	491.7	645.5
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	0.0
Fixed Maturities [Member] | Government Sponsored Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	111.1	90.3
Gross Unrealized Gains	0.1	0.3
Gross Unrealized Losses, Less than 12 Months	(0.8)	(0.1)
Gross Unrealized Losses, 12 Months or Greater	(0.5)	(0.4)
Available-for-sale Securities	109.9	90.1
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	0.0
Fixed Maturities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Investments [Line Items]
Cost or Amortized Cost	5,096.5	5,854.6
Gross Unrealized Gains	98.0	192.6
Gross Unrealized Losses, Less than 12 Months	(19.0)	(5.0)
Gross Unrealized Losses, 12 Months or Greater	(10.5)	(7.3)
Available-for-sale Securities	5,165.0	6,034.9
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	0.0
Fixed Maturities [Member] | Corporate Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	7,891.3	7,362.8
Gross Unrealized Gains	55.2	165.8
Gross Unrealized Losses, Less than 12 Months	(140.2)	(30.2)
Gross Unrealized Losses, 12 Months or Greater	(23.8)	(12.6)
Available-for-sale Securities	7,782.5	7,485.8
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	(0.3)
Fixed Maturities [Member] | Residential Mortgage-Backed Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	2,832.2	2,520.0
Gross Unrealized Gains	29.8	38.5
Gross Unrealized Losses, Less than 12 Months	(38.2)	(8.0)
Gross Unrealized Losses, 12 Months or Greater	(23.2)	(11.6)
Available-for-sale Securities	2,800.6	2,538.9
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	0.0
Fixed Maturities [Member] | Commercial Mortgage-Backed Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	75.6	80.1
Gross Unrealized Gains	0.2	0.7
Gross Unrealized Losses, Less than 12 Months	(0.5)	(0.1)
Gross Unrealized Losses, 12 Months or Greater	(2.0)	(2.0)
Available-for-sale Securities	73.3	78.7
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	0.0
Fixed Maturities [Member] | Other Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	1,137.7	1,053.7
Gross Unrealized Gains	15.6	14.4
Gross Unrealized Losses, Less than 12 Months	(5.8)	(2.4)
Gross Unrealized Losses, 12 Months or Greater	(1.4)	(1.5)
Available-for-sale Securities	1,146.1	1,064.2
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	$ 0.0	$ 0.0