Investments (Current And Long-Term Investments, Available-For-Sale) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2016	Dec. 31, 2015
Investments [Line Items]
Cost or Amortized Cost	$ 19,782.0	$ 18,583.5
Gross Unrealized Gains	1,099.0	815.0
Gross Unrealized Losses, Less than 12 Months	(55.0)	(321.5)
Gross Unrealized Losses, 12 Months or Greater	(86.4)	(126.0)
Available-for-sale Securities	20,739.6	18,951.0
Equity Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	1,331.1	1,083.1
Gross Unrealized Gains	352.2	420.6
Gross Unrealized Losses, Less than 12 Months	(18.9)	(30.9)
Gross Unrealized Losses, 12 Months or Greater	0.0	0.0
Available-for-sale Securities	1,664.4	1,472.8
Fixed Maturities [Member]
Investments [Line Items]
Cost or Amortized Cost	18,450.9	17,500.4
Gross Unrealized Gains	746.8	394.4
Gross Unrealized Losses, Less than 12 Months	(36.1)	(290.6)
Gross Unrealized Losses, 12 Months or Greater	(86.4)	(126.0)
Available-for-sale Securities	19,075.2	17,478.2
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(26.4)	(15.4)
Fixed Maturities [Member] \| United States Government Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	414.4	349.5
Gross Unrealized Gains	14.0	2.0
Gross Unrealized Losses, Less than 12 Months	0.0	(1.6)
Gross Unrealized Losses, 12 Months or Greater	0.0	0.0
Available-for-sale Securities	428.4	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	1.2	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	51.0	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	5,924.6	5,976.7
Gross Unrealized Gains	410.0	284.1
Gross Unrealized Losses, Less than 12 Months	(0.4)	(4.0)
Gross Unrealized Losses, 12 Months or Greater	(4.5)	(5.2)
Available-for-sale Securities	6,329.7	6,251.6
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	9,152.2	8,209.7
Gross Unrealized Gains	256.3	61.1
Gross Unrealized Losses, Less than 12 Months	(29.8)	(267.2)
Gross Unrealized Losses, 12 Months or Greater	(68.8)	(110.5)
Available-for-sale Securities	9,309.9	7,893.1
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(26.4)	(15.4)
Fixed Maturities [Member] \| Residential Mortgage-Backed Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	1,835.4	1,724.5
Gross Unrealized Gains	57.3	41.2
Gross Unrealized Losses, Less than 12 Months	(1.8)	(7.6)
Gross Unrealized Losses, 12 Months or Greater	(4.8)	(7.2)
Available-for-sale Securities	1,886.1	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	330.5	407.6
Gross Unrealized Gains	3.8	1.4
Gross Unrealized Losses, Less than 12 Months	(0.6)	(4.3)
Gross Unrealized Losses, 12 Months or Greater	(3.1)	(0.4)
Available-for-sale Securities	330.6	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	744.0	756.8
Gross Unrealized Gains	4.2	4.1
Gross Unrealized Losses, Less than 12 Months	(3.5)	(5.8)
Gross Unrealized Losses, 12 Months or Greater	(5.2)	(2.6)
Available-for-sale Securities	739.5	752.5
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	$ 0.0	$ 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
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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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