Investments (Current And Long-Term Investments, Available-For-Sale) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Investments [Line Items]
Cost or Amortized Cost	$ 19,854.7	$ 18,619.7
Gross Unrealized Gains	940.9	748.6
Gross Unrealized Losses, Less than 12 Months	(47.4)	(138.5)
Gross Unrealized Losses, 12 Months or Greater	(32.7)	(42.4)
Available-for-sale Securities	20,715.5	19,187.4
Equity Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	1,061.0	1,103.3
Gross Unrealized Gains	439.9	407.3
Gross Unrealized Losses, Less than 12 Months	(16.0)	(10.7)
Gross Unrealized Losses, 12 Months or Greater	0.0	0.0
Available-for-sale Securities	1,484.9	1,499.9
Fixed Maturities [Member]
Investments [Line Items]
Cost or Amortized Cost	18,793.7	17,516.4
Gross Unrealized Gains	501.0	341.3
Gross Unrealized Losses, Less than 12 Months	(31.4)	(127.8)
Gross Unrealized Losses, 12 Months or Greater	(32.7)	(42.4)
Available-for-sale Securities	19,230.6	17,687.5
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(0.2)	(7.2)
Fixed Maturities [Member] \| United States Government Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	578.4	561.7
Gross Unrealized Gains	1.2	2.5
Gross Unrealized Losses, Less than 12 Months	(2.7)	(5.7)
Gross Unrealized Losses, 12 Months or Greater	(0.4)	0.0
Available-for-sale Securities	576.5	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	71.3	40.1
Gross Unrealized Gains	0.3	0.3
Gross Unrealized Losses, Less than 12 Months	(0.2)	(0.3)
Gross Unrealized Losses, 12 Months or Greater	(0.1)	(0.1)
Available-for-sale Securities	71.3	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	5,675.4	6,024.6
Gross Unrealized Gains	207.5	139.1
Gross Unrealized Losses, Less than 12 Months	(2.4)	(55.2)
Gross Unrealized Losses, 12 Months or Greater	(8.0)	(3.2)
Available-for-sale Securities	5,872.5	6,105.3
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	0.0	(3.8)
Fixed Maturities [Member] \| Corporate Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	8,967.2	8,011.7
Gross Unrealized Gains	234.3	159.5
Gross Unrealized Losses, Less than 12 Months	(18.4)	(49.5)
Gross Unrealized Losses, 12 Months or Greater	(15.2)	(27.1)
Available-for-sale Securities	9,167.9	8,094.6
Non-Credit Component of Other-Than-Temporary Impairments Recognized in Accumulated Other Comprehensive Income	(0.2)	(3.4)
Fixed Maturities [Member] \| Residential Mortgage-Backed Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	2,488.0	1,916.9
Gross Unrealized Gains	42.7	32.3
Gross Unrealized Losses, Less than 12 Months	(7.0)	(15.3)
Gross Unrealized Losses, 12 Months or Greater	(5.5)	(4.6)
Available-for-sale Securities	2,518.2	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	79.1	216.8
Gross Unrealized Gains	0.9	1.2
Gross Unrealized Losses, Less than 12 Months	0.0	(0.3)
Gross Unrealized Losses, 12 Months or Greater	(2.1)	(3.4)
Available-for-sale Securities	77.9	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	934.3	744.6
Gross Unrealized Gains	14.1	6.4
Gross Unrealized Losses, Less than 12 Months	(0.7)	(1.5)
Gross Unrealized Losses, 12 Months or Greater	(1.4)	(4.0)
Available-for-sale Securities	946.3	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.

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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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