Investments (Current And Long-Term Investments, Available-For-Sale) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Cost or Amortized Cost	$ 20,735.3	$ 18,619.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	939.7	748.6
Gross Unrealized Gains	939.7	748.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(68.8)	(138.5)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(36.1)	(42.4)
Estimated Fair Value	21,570.1	19,187.4
Fixed Maturity Securities [Member]
Investment [Line Items]
Cost or Amortized Cost	17,610.5	17,516.4
Gross Unrealized Gains	414.5	341.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(50.8)	(127.8)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(36.1)	(42.4)
Estimated Fair Value	17,938.1	17,687.5
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(0.3)	(7.2)
Fixed Maturity Securities [Member] | United States Government Securities [Member]
Investment [Line Items]
Cost or Amortized Cost	649.0	561.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2.2	2.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(5.0)	(5.7)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(0.7)	0.0
Estimated Fair Value	645.5	558.5
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	0.0
Fixed Maturity Securities [Member] | Government Sponsored Securities [Member]
Investment [Line Items]
Cost or Amortized Cost	90.3	40.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0.3	0.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(0.1)	(0.3)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(0.4)	(0.1)
Estimated Fair Value	90.1	40.0
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	0.0
Fixed Maturity Securities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Investment [Line Items]
Cost or Amortized Cost	5,854.6	6,024.6
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	192.6	139.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(5.0)	(55.2)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(7.3)	(3.2)
Estimated Fair Value	6,034.9	6,105.3
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	(3.8)
Fixed Maturity Securities [Member] | Corporate Securities [Member]
Investment [Line Items]
Cost or Amortized Cost	7,362.8	8,011.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	165.8	159.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(30.2)	(49.5)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(12.6)	(27.1)
Estimated Fair Value	7,485.8	8,094.6
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(0.3)	(3.4)
Fixed Maturity Securities [Member] | Residential Mortgage-Backed Securities [Member]
Investment [Line Items]
Cost or Amortized Cost	2,520.0	1,916.9
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	38.5	32.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(8.0)	(15.3)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(11.6)	(4.6)
Estimated Fair Value	2,538.9	1,929.3
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	0.0
Fixed Maturity Securities [Member] | Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Cost or Amortized Cost	80.1	216.8
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0.7	1.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(0.1)	(0.3)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2.0)	(3.4)
Estimated Fair Value	78.7	214.3
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	0.0
Fixed Maturity Securities [Member] | Other Security Investments [Member]
Investment [Line Items]
Cost or Amortized Cost	1,053.7	744.6
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	14.4	6.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(2.4)	(1.5)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1.5)	(4.0)
Estimated Fair Value	1,064.2	745.5
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	0.0
Equity Securities [Member]
Investment [Line Items]
Cost or Amortized Cost	3,124.8	1,103.3
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	525.2	407.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(18.0)	(10.7)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	0.0
Estimated Fair Value	$ 3,632.0	$ 1,499.9