Investments (Current And Long-Term Investments, Available-For-Sale) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
Total available-for-sale fixed maturity securities, Amortized Cost	$ 16,587.1	$ 16,440.5
Gross Unrealized Gains	1,190.3	1,107.1
Gross Unrealized Losses, Less than 12 Months	(50.6)	(129.2)
Gross Unrealized Losses, Greater than 12 Months	(31.3)	(41.6)
Estimated Fair Value	17,695.5	17,376.8
Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Total available-for-sale fixed maturity securities, Amortized Cost	15,689.3	15,474.4
Gross Unrealized Gains	827.3	830.1
Gross Unrealized Losses, Less than 12 Months	(43.5)	(103.0)
Gross Unrealized Losses, Greater than 12 Months	(31.3)	(41.6)
Estimated Fair Value	16,441.8	16,159.9
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	2.8	10.3
Equity Securities [Member]
Schedule of Investments [Line Items]
Total available-for-sale fixed maturity securities, Amortized Cost	897.8	966.1
Gross Unrealized Gains	363.0	277.0
Gross Unrealized Losses, Less than 12 Months	(7.1)	(26.2)
Gross Unrealized Losses, Greater than 12 Months	0	0
Estimated Fair Value	1,253.7	1,216.9
United States Government Securities [Member]
Schedule of Investments [Line Items]
Total available-for-sale fixed maturity securities, Amortized Cost	547.7
United States Government Securities [Member] | Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Total available-for-sale fixed maturity securities, Amortized Cost		564.9
Gross Unrealized Gains	21.8	39.9
Gross Unrealized Losses, Less than 12 Months	(1.6)	(0.1)
Gross Unrealized Losses, Greater than 12 Months	0	0
Estimated Fair Value	567.9	604.7
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]
Total available-for-sale fixed maturity securities, Amortized Cost	116.4	173.1
Gross Unrealized Gains	1.9	2.5
Gross Unrealized Losses, Less than 12 Months	(0.1)	0
Gross Unrealized Losses, Greater than 12 Months	0	0
Estimated Fair Value	118.2	175.6
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]
Total available-for-sale fixed maturity securities, Amortized Cost	4,930.1	4,994.2
Gross Unrealized Gains	341.2	352.3
Gross Unrealized Losses, Less than 12 Months	(6.3)	(3.9)
Gross Unrealized Losses, Greater than 12 Months	(8.4)	(15.0)
Estimated Fair Value	5,256.6	5,327.6
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	0	0.5
Corporate Securities [Member] | Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Total available-for-sale fixed maturity securities, Amortized Cost	7,082.7	6,588.0
Gross Unrealized Gains	333.8	305.3
Gross Unrealized Losses, Less than 12 Months	(31.1)	(88.4)
Gross Unrealized Losses, Greater than 12 Months	(7.7)	(6.9)
Estimated Fair Value	7,377.7	6,798.0
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	1.1	0.4
Options Embedded In Convertible Debt Securities [Member] | Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Total available-for-sale fixed maturity securities, Amortized Cost	90.0	79.7
Gross Unrealized Gains	0	0
Gross Unrealized Losses, Less than 12 Months	0	0
Gross Unrealized Losses, Greater than 12 Months	0	0
Estimated Fair Value	90.0	79.7
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]
Total available-for-sale fixed maturity securities, Amortized Cost	2,301.2	2,471.4
Gross Unrealized Gains	105.7	112.1
Gross Unrealized Losses, Less than 12 Months	(2.1)	(7.6)
Gross Unrealized Losses, Greater than 12 Months	(6.9)	(10.9)
Estimated Fair Value	2,397.9	2,565.0
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	0.4	6.2
Commercial Mortgage-Backed Securities [Member] | Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Total available-for-sale fixed maturity securities, Amortized Cost	364.6	363.2
Gross Unrealized Gains	19.0	14.9
Gross Unrealized Losses, Less than 12 Months	(1.0)	(1.0)
Gross Unrealized Losses, Greater than 12 Months	(2.0)	(1.7)
Estimated Fair Value	380.6	375.4
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	0	0
Other Debt Obligations [Member] | Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Total available-for-sale fixed maturity securities, Amortized Cost	256.6	239.9
Gross Unrealized Gains	3.9	3.1
Gross Unrealized Losses, Less than 12 Months	(1.3)	(2.0)
Gross Unrealized Losses, Greater than 12 Months	(6.3)	(7.1)
Estimated Fair Value	252.9	233.9
Non-Credit Component of Other-Than-Temporary Impairments Recognized in AOCI	$ 1.3	$ 3.2