Investments (Current And Long-Term Investments, Available-For-Sale) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Schedule of Investments [Line Items]
Cost or Amortized Cost	$ 18,764.7	$ 17,356.1
Gross Unrealized Gains	1,003.0	1,282.1	1,373.9
Gross Unrealized Losses, Less Than 12 Months	(258.5)	(36.7)
Gross Unrealized Losses, 12 Months or Greater	(18.2)	(14.6)
Total available-for-sale fixed maturity securities, Estimated Fair Value	19,491.0	18,586.9
Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	17,714.2	16,459.1
Gross Unrealized Gains	521.6	924.1
Gross Unrealized Losses, Less Than 12 Months	(241.8)	(24.2)
Gross Unrealized Losses, 12 Months or Greater	(18.2)	(14.6)
Total available-for-sale fixed maturity securities, Estimated Fair Value	17,975.8	17,344.4
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	(0.9)	(3.4)
Equity Securities [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	1,050.5	897.0
Gross Unrealized Gains	481.4	358.0
Gross Unrealized Losses, Less Than 12 Months	(16.7)	(12.5)
Gross Unrealized Losses, 12 Months or Greater	0	0
Total available-for-sale fixed maturity securities, Estimated Fair Value	1,515.2	1,242.5
United States Government Securities [Member] | Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	464.0	330.3
Gross Unrealized Gains	4.1	13.1
Gross Unrealized Losses, Less Than 12 Months	(1.4)	(0.2)
Gross Unrealized Losses, 12 Months or Greater	0	0
Total available-for-sale fixed maturity securities, Estimated Fair Value	466.7	343.2
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	0	0
Government Sponsored Securities [Member] | Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	195.2	153.6
Gross Unrealized Gains	0.9	2.6
Gross Unrealized Losses, Less Than 12 Months	(1.5)	0
Gross Unrealized Losses, 12 Months or Greater	0	0
Total available-for-sale fixed maturity securities, Estimated Fair Value	194.6	156.2
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	0	0
States, Municipalities And Political Subdivisions, Tax-Exempt [Member] | Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	6,143.2	5,501.3
Gross Unrealized Gains	234.9	388.2
Gross Unrealized Losses, Less Than 12 Months	(93.6)	(5.7)
Gross Unrealized Losses, 12 Months or Greater	(1.2)	(1.6)
Total available-for-sale fixed maturity securities, Estimated Fair Value	6,283.3	5,882.2
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	0	0
Corporate Securities [Member] | Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	7,780.3	7,642.0
Gross Unrealized Gains	212.0	387.0
Gross Unrealized Losses, Less Than 12 Months	(109.3)	(17.0)
Gross Unrealized Losses, 12 Months or Greater	(12.4)	(8.0)
Total available-for-sale fixed maturity securities, Estimated Fair Value	7,870.6	8,004.0
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	(0.1)	(1.7)
Options Embedded In Convertible Debt Securities [Member] | Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	81.0	67.2
Gross Unrealized Gains	0	0
Gross Unrealized Losses, Less Than 12 Months	0	0
Gross Unrealized Losses, 12 Months or Greater	0	0
Total available-for-sale fixed maturity securities, Estimated Fair Value	81.0	67.2
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	0	0
Residential Mortgage-Backed Securities [Member] | Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	2,198.4	2,204.7
Gross Unrealized Gains	52.6	103.1
Gross Unrealized Losses, Less Than 12 Months	(29.4)	(1.1)
Gross Unrealized Losses, 12 Months or Greater	(3.3)	(1.9)
Total available-for-sale fixed maturity securities, Estimated Fair Value	2,218.3	2,304.8
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	0	(0.4)
Commercial Mortgage-Backed Securities [Member] | Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	488.1	323.2
Gross Unrealized Gains	11.3	22.5
Gross Unrealized Losses, Less Than 12 Months	(3.4)	0
Gross Unrealized Losses, 12 Months or Greater	0	0
Total available-for-sale fixed maturity securities, Estimated Fair Value	496.0	345.7
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	0	0
Other Debt Securities [Member] | Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	364.0	236.8
Gross Unrealized Gains	5.8	7.6
Gross Unrealized Losses, Less Than 12 Months	(3.2)	(0.2)
Gross Unrealized Losses, 12 Months or Greater	(1.3)	(3.1)
Total available-for-sale fixed maturity securities, Estimated Fair Value	365.3	241.1
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	$ (0.8)	$ (1.3)