Investments (Current And Long-Term Investments, Available-For-Sale) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2016	Dec. 31, 2015
Investments [Line Items]
Cost or Amortized Cost	$ 19,286.9	$ 18,583.5
Gross Unrealized Gains	1,068.9	815.0
Gross Unrealized Losses, Less than 12 Months	(37.3)	(321.5)
Gross Unrealized Losses, 12 Months or Greater	(49.0)	(126.0)
Available-for-sale Securities	20,269.5	18,951.0
Equity Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	1,170.4	1,083.1
Gross Unrealized Gains	357.9	420.6
Gross Unrealized Losses, Less than 12 Months	(16.3)	(30.9)
Gross Unrealized Losses, 12 Months or Greater	0.0	0.0
Available-for-sale Securities	1,512.0	1,472.8
Fixed Maturities [Member]
Investments [Line Items]
Cost or Amortized Cost	18,116.5	17,500.4
Gross Unrealized Gains	711.0	394.4
Gross Unrealized Losses, Less than 12 Months	(21.0)	(290.6)
Gross Unrealized Losses, 12 Months or Greater	(49.0)	(126.0)
Available-for-sale Securities	18,757.5	17,478.2
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(12.1)	(15.4)
Fixed Maturities [Member] \| United States Government Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	461.1	349.5
Gross Unrealized Gains	10.6	2.0
Gross Unrealized Losses, Less than 12 Months	(0.8)	(1.6)
Gross Unrealized Losses, 12 Months or Greater	0.0	0.0
Available-for-sale Securities	470.9	349.9
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	49.8	75.6
Gross Unrealized Gains	0.9	0.5
Gross Unrealized Losses, Less than 12 Months	0.0	(0.1)
Gross Unrealized Losses, 12 Months or Greater	0.0	(0.1)
Available-for-sale Securities	50.7	75.9
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	5,983.7	5,976.7
Gross Unrealized Gains	345.0	284.1
Gross Unrealized Losses, Less than 12 Months	(3.2)	(4.0)
Gross Unrealized Losses, 12 Months or Greater	(3.0)	(5.2)
Available-for-sale Securities	6,322.5	6,251.6
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(1.4)	0.0
Fixed Maturities [Member] \| Corporate Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	8,613.4	8,209.7
Gross Unrealized Gains	287.3	61.1
Gross Unrealized Losses, Less than 12 Months	(14.7)	(267.2)
Gross Unrealized Losses, 12 Months or Greater	(37.0)	(110.5)
Available-for-sale Securities	8,849.0	7,893.1
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(10.1)	(15.4)
Fixed Maturities [Member] \| Residential Mortgage-Backed Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	1,953.6	1,724.5
Gross Unrealized Gains	56.3	41.2
Gross Unrealized Losses, Less than 12 Months	(1.1)	(7.6)
Gross Unrealized Losses, 12 Months or Greater	(3.0)	(7.2)
Available-for-sale Securities	2,005.8	1,750.9
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	284.5	407.6
Gross Unrealized Gains	3.3	1.4
Gross Unrealized Losses, Less than 12 Months	(0.1)	(4.3)
Gross Unrealized Losses, 12 Months or Greater	(2.6)	(0.4)
Available-for-sale Securities	285.1	404.3
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	0.0
Fixed Maturities [Member] \| Other Debt Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	770.4	756.8
Gross Unrealized Gains	7.6	4.1
Gross Unrealized Losses, Less than 12 Months	(1.1)	(5.8)
Gross Unrealized Losses, 12 Months or Greater	(3.4)	(2.6)
Available-for-sale Securities	773.5	752.5
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	$ (0.6)	$ 0.0

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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Amount before tax of other than temporary impairment (OTTI) on investment in available-for-sale debt security, recognized in other comprehensive loss, attributable to parent entity.

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