Investments (Current And Long-Term Investments, Available-For-Sale) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2017	Dec. 31, 2016
Investments [Line Items]
Cost or Amortized Cost	$ 19,668.4	$ 18,619.7
Gross Unrealized Gains	811.0	748.6
Gross Unrealized Losses, Less than 12 Months	(96.3)	(138.5)
Gross Unrealized Losses, 12 Months or Greater	(28.0)	(42.4)
Available-for-sale Securities	20,355.1	19,187.4
Equity Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	1,417.6	1,103.3
Gross Unrealized Gains	441.4	407.3
Gross Unrealized Losses, Less than 12 Months	(13.6)	(10.7)
Gross Unrealized Losses, 12 Months or Greater	0.0	0.0
Available-for-sale Securities	1,845.4	1,499.9
Fixed Maturities [Member]
Investments [Line Items]
Cost or Amortized Cost	18,250.8	17,516.4
Gross Unrealized Gains	369.6	341.3
Gross Unrealized Losses, Less than 12 Months	(82.7)	(127.8)
Gross Unrealized Losses, 12 Months or Greater	(28.0)	(42.4)
Available-for-sale Securities	18,509.7	17,687.5
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(1.5)	(7.2)
Fixed Maturities [Member] \| United States Government Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	494.1	561.7
Gross Unrealized Gains	2.3	2.5
Gross Unrealized Losses, Less than 12 Months	(3.0)	(5.7)
Gross Unrealized Losses, 12 Months or Greater	0.0	0.0
Available-for-sale Securities	493.4	558.5
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	86.8	40.1
Gross Unrealized Gains	0.3	0.3
Gross Unrealized Losses, Less than 12 Months	(0.3)	(0.3)
Gross Unrealized Losses, 12 Months or Greater	(0.1)	(0.1)
Available-for-sale Securities	86.7	40.0
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	5,441.5	6,024.6
Gross Unrealized Gains	153.3	139.1
Gross Unrealized Losses, Less than 12 Months	(31.0)	(55.2)
Gross Unrealized Losses, 12 Months or Greater	(2.1)	(3.2)
Available-for-sale Securities	5,561.7	6,105.3
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(1.4)	(3.8)
Fixed Maturities [Member] \| Corporate Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	9,422.2	8,011.7
Gross Unrealized Gains	174.1	159.5
Gross Unrealized Losses, Less than 12 Months	(32.4)	(49.5)
Gross Unrealized Losses, 12 Months or Greater	(15.8)	(27.1)
Available-for-sale Securities	9,548.1	8,094.6
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(0.1)	(3.4)
Fixed Maturities [Member] \| Residential Mortgage-Backed Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	1,914.0	1,916.9
Gross Unrealized Gains	31.0	32.3
Gross Unrealized Losses, Less than 12 Months	(15.1)	(15.3)
Gross Unrealized Losses, 12 Months or Greater	(4.1)	(4.6)
Available-for-sale Securities	1,925.8	1,929.3
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	145.1	216.8
Gross Unrealized Gains	0.9	1.2
Gross Unrealized Losses, Less than 12 Months	(0.3)	(0.3)
Gross Unrealized Losses, 12 Months or Greater	(2.9)	(3.4)
Available-for-sale Securities	142.8	214.3
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	747.1	744.6
Gross Unrealized Gains	7.7	6.4
Gross Unrealized Losses, Less than 12 Months	(0.6)	(1.5)
Gross Unrealized Losses, 12 Months or Greater	(3.0)	(4.0)
Available-for-sale Securities	751.2	745.5
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
-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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