Investments (Current And Long-Term Investments, Available-For-Sale) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Cost or Amortized Cost	$ 18,619.7	$ 18,583.5
Gross Unrealized Gains	748.6	815.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(138.5)	(321.5)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(42.4)	(126.0)
Estimated Fair Value	19,187.4	18,951.0
Fixed Maturity Securities [Member]
Investment [Line Items]
Cost or Amortized Cost	17,516.4	17,500.4
Gross Unrealized Gains	341.3	394.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(127.8)	(290.6)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(42.4)	(126.0)
Estimated Fair Value	17,687.5	17,478.2
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(7.2)	(15.4)
Fixed Maturity Securities [Member] | United States Government Securities [Member]
Investment [Line Items]
Cost or Amortized Cost	561.7	349.5
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2.5	2.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(5.7)	(1.6)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	0.0
Estimated Fair Value	558.5	349.9
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	40.1	75.6
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0.3	0.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(0.3)	(0.1)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(0.1)	(0.1)
Estimated Fair Value	40.0	75.9
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	6,024.6	5,976.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	139.1	284.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(55.2)	(4.0)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(3.2)	(5.2)
Estimated Fair Value	6,105.3	6,251.6
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(3.8)	0.0
Fixed Maturity Securities [Member] | Corporate Securities [Member]
Investment [Line Items]
Cost or Amortized Cost	8,011.7	8,209.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	159.5	61.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(49.5)	(267.2)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(27.1)	(110.5)
Estimated Fair Value	8,094.6	7,893.1
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(3.4)	(15.4)
Fixed Maturity Securities [Member] | Residential Mortgage-Backed Securities [Member]
Investment [Line Items]
Cost or Amortized Cost	1,916.9	1,724.5
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	32.3	41.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(15.3)	(7.6)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(4.6)	(7.2)
Estimated Fair Value	1,929.3	1,750.9
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	216.8	407.6
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1.2	1.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(0.3)	(4.3)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(3.4)	(0.4)
Estimated Fair Value	214.3	404.3
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	0.0
Fixed Maturity Securities [Member] | Other Debt Securities [Member]
Investment [Line Items]
Cost or Amortized Cost	744.6	756.8
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	6.4	4.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1.5)	(5.8)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(4.0)	(2.6)
Estimated Fair Value	745.5	752.5
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	1,103.3	1,083.1
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	407.3	420.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(10.7)	(30.9)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	0.0
Estimated Fair Value	$ 1,499.9	$ 1,472.8