Investments (Current And Long-Term Investments, Available-For-Sale) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Investments [Line Items]
Cost or Amortized Cost	$ 19,506.2		$ 18,478.5
Gross Unrealized Gains	1,156.5	1,324.3	1,054.2
Gross Unrealized Losses, Less than 12 Months	(131.2)		(244.5)
Gross Unrealized Losses, 12 Months or Greater	(30.6)		(33.3)
Estimated Fair Value	20,500.9		19,254.9
Equity Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	1,400.8		1,195.9
Gross Unrealized Gains	604.3		578.9
Gross Unrealized Losses, Less than 12 Months	(10.6)		(8.0)
Gross Unrealized Losses, 12 Months or Greater	0		0
Estimated Fair Value	1,994.5		1,766.8
Fixed Maturities [Member]
Investments [Line Items]
Cost or Amortized Cost	18,105.4		17,282.6
Gross Unrealized Gains	552.2		475.3
Gross Unrealized Losses, Less than 12 Months	(120.6)		(236.5)
Gross Unrealized Losses, 12 Months or Greater	(30.6)		(33.3)
Estimated Fair Value	18,506.4		17,488.1
Non-credit component of other-than-temporary impairments recognized in accumulated other comprehensive income	0		(0.8)
Fixed Maturities [Member] | United States Government Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	460.2		300.8
Gross Unrealized Gains	2.7		2.5
Gross Unrealized Losses, Less than 12 Months	(1.3)		(3.4)
Gross Unrealized Losses, 12 Months or Greater	0		0
Estimated Fair Value	461.6		299.9
Non-credit component of other-than-temporary impairments recognized in accumulated other comprehensive income	0		0
Fixed Maturities [Member] | Government Sponsored Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	168.6		174.4
Gross Unrealized Gains	0.6		0.4
Gross Unrealized Losses, Less than 12 Months	(0.5)		(1.3)
Gross Unrealized Losses, 12 Months or Greater	(0.5)		0
Estimated Fair Value	168.2		173.5
Non-credit component of other-than-temporary impairments recognized in accumulated other comprehensive income	0		0
Fixed Maturities [Member] | States, Municipalities And Political Subdivisions, Tax-Exempt [Member]
Investments [Line Items]
Cost or Amortized Cost	5,801.9		5,899.5
Gross Unrealized Gains	244.4		202.9
Gross Unrealized Losses, Less than 12 Months	(33.3)		(90.1)
Gross Unrealized Losses, 12 Months or Greater	(9.5)		(9.6)
Estimated Fair Value	6,003.5		6,002.7
Non-credit component of other-than-temporary impairments recognized in accumulated other comprehensive income	0		(0.6)
Fixed Maturities [Member] | Corporate Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	8,338.4		7,614.1
Gross Unrealized Gains	232.4		205.2
Gross Unrealized Losses, Less than 12 Months	(54.8)		(95.2)
Gross Unrealized Losses, 12 Months or Greater	(14.1)		(15.5)
Estimated Fair Value	8,501.9		7,708.6
Non-credit component of other-than-temporary impairments recognized in accumulated other comprehensive income	0		(0.1)
Fixed Maturities [Member] | Options Embedded In Convertible Debt Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	97.6		89.2
Gross Unrealized Gains	0		0
Gross Unrealized Losses, Less than 12 Months	0		0
Gross Unrealized Losses, 12 Months or Greater	0		0
Estimated Fair Value	97.6		89.2
Non-credit component of other-than-temporary impairments recognized in accumulated other comprehensive income	0		0
Fixed Maturities [Member] | Residential Mortgage-Backed Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	2,160.0		2,269.4
Gross Unrealized Gains	53.9		48.0
Gross Unrealized Losses, Less than 12 Months	(26.8)		(41.4)
Gross Unrealized Losses, 12 Months or Greater	(5.5)		(7.1)
Estimated Fair Value	2,181.6		2,268.9
Non-credit component of other-than-temporary impairments recognized in accumulated other comprehensive income	0		0
Fixed Maturities [Member] | Commercial Mortgage-Backed Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	509.4		479.0
Gross Unrealized Gains	11.1		10.5
Gross Unrealized Losses, Less than 12 Months	(1.1)		(2.6)
Gross Unrealized Losses, 12 Months or Greater	(0.2)		(0.3)
Estimated Fair Value	519.2		486.6
Non-credit component of other-than-temporary impairments recognized in accumulated other comprehensive income	0		0
Fixed Maturities [Member] | Other Debt Securities [Member]
Investments [Line Items]
Cost or Amortized Cost	569.3		456.2
Gross Unrealized Gains	7.1		5.8
Gross Unrealized Losses, Less than 12 Months	(2.8)		(2.5)
Gross Unrealized Losses, 12 Months or Greater	(0.8)		(0.8)
Estimated Fair Value	572.8		458.7
Non-credit component of other-than-temporary impairments recognized in accumulated other comprehensive income	$ 0		$ (0.1)