(Rollforward of Fair Value Measurements by Level) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Dec. 31, 2011		Dec. 31, 2010
Level 1 Inputs
Investment securities
Fair value investments	$ 2,225.8		$ 1,755.2		$ 1,879.1		$ 1,894.4
Total realized and unrealized gains (losses)	57.8		(50.5)
Foreign currency gains (losses) through OCI	13.2		0.3
Amortization/Accretion	(0.7)		2.4
Purchases	6,712.0		6,690.7
Sales	(6,435.6)		(6,782.1)
Transfers in	0		0
Transfers out	0		0
Level 2 Inputs
Investment securities
Fair value investments	5,016.0		4,969.5		6,088.2		5,477.4
Total realized and unrealized gains (losses)	53.8		115.9
Foreign currency gains (losses) through OCI	75.5		(84.4)
Amortization/Accretion	(34.0)		(40.7)
Purchases	3,823.7		3,987.4
Sales	(5,048.1)		(4,596.9)
Transfers in	56.9		111.8
Transfers out	0		(1.0)
Securities of level 3 measurements recategorized as level 2 measurements	56.9		111.8
Level 3 Inputs
Investment securities
Fair value investments							0
Total realized and unrealized gains (losses)			0
Foreign currency gains (losses) through OCI			0
Amortization/Accretion			0
Purchases			0
Sales			0
Fair Value
Investment securities
Fair value investments	7,626.4	[1]	7,265.9	[1]	8,346.8	[1],[2]	7,901.6	[1]
Total realized and unrealized gains (losses)	123.2		72.4
Foreign currency gains (losses) through OCI	92.6		(91.9)
Amortization/Accretion	(35.4)		(38.4)
Purchases	10,723.1		10,938.5
Sales	(11,623.9)		(11,516.3)
Transfers in	56.9		112.8
Transfers out	(56.9)		(112.8)
Fixed maturity investments.
Investment securities
Difference in value between securities determined using the industry standard pricing models and securities based upon quoted prices provided by a third party			0.8
Fixed maturity investments. | Level 3 Inputs
Investment securities
Fair value investments	76.1		216.9		78.9		128.4
Total realized and unrealized gains (losses)	8.4		(10.6)
Foreign currency gains (losses) through OCI	0.7		(4.4)
Amortization/Accretion	(0.7)		(0.1)
Purchases	144.3		212.8
Sales	(99.2)		1.6
Transfers in	0		1.0
Transfers out	(56.3)		(111.8)
Common equity securities | Level 3 Inputs
Investment securities
Fair value investments	35.9		32.1		32.3		71.2
Total realized and unrealized gains (losses)	11.4		(4.9)
Foreign currency gains (losses) through OCI	0.1		1.6
Amortization/Accretion	0		0
Purchases	2.5		19.7
Sales	(9.8)		(55.5)
Transfers in	0		0
Transfers out	(0.6)		0
Convertible Debt Securities [Member] | Level 3 Inputs
Investment securities
Fair value investments	0		0		0
Total realized and unrealized gains (losses)	0
Foreign currency gains (losses) through OCI	0
Amortization/Accretion	0
Purchases	0
Sales	0
Transfers in	0		0
Transfers out	0		0
Other long-term investments | Level 3 Inputs
Investment securities
Fair value investments	272.6	[1]	292.2	[1]	268.3	[1]	330.2	[1]
Total realized and unrealized gains (losses)	(8.2)		22.5
Foreign currency gains (losses) through OCI	3.1		(5.0)
Amortization/Accretion	0		0
Purchases	40.6		27.9
Sales	(31.2)		(83.4)
Transfers in	0		0
Transfers out	0		0
Fair value measured on a recurring basis | Level 1 Inputs
Investment securities
Fair value investments	2,225.8				1,879.1
Fair value measured on a recurring basis | Level 2 Inputs
Investment securities
Fair value investments	5,016.0				6,088.2
Fair value measured on a recurring basis | Level 3 Inputs
Investment securities
Fair value investments	384.6				379.5
Fair value measured on a recurring basis | Fair Value
Investment securities
Fair value investments	7,626.4				8,346.8
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs
Investment securities
Fair value investments	398.5				365.7	[3]
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs
Investment securities
Fair value investments	4,815.1				5,889.1	[3]
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs
Investment securities
Fair value investments	76.1				78.9	[3]
Fair value measured on a recurring basis | Fixed maturity investments. | Fair Value
Investment securities
Fair value investments	5,289.7				6,333.7	[3]
Fair value measured on a recurring basis | Convertible Debt Securities [Member] | Level 1 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Convertible Debt Securities [Member] | Level 2 Inputs
Investment securities
Fair value investments	142.0				143.8
Fair value measured on a recurring basis | Convertible Debt Securities [Member] | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Convertible Debt Securities [Member] | Fair Value
Investment securities
Fair value investments	142.0				143.8
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Investment securities
Fair value investments	0	[3]			0	[4]
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Investment securities
Fair value investments	0	[3]			0	[4]
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Investment securities
Fair value investments	272.6	[3]			268.3	[4]
Carrying value of investment accounted for using the equity method	34.3		36.6		33.0		41.9
Fair value measured on a recurring basis | Other long-term investments | Fair Value
Investment securities
Fair value investments	$ 272.6	[3]			$ 268.3	[4]

[1] (1) Excludes carrying value of $33.0 and $34.3 at January 1, 2012 and September 30, 2012 associated with other long-term investments accounted for using the equity method

[2] (2) Carrying value includes $111.8 and $377.3 at January 1, 2012 and September 30, 2012 that is classified as assets held for sale relating to discontinued operations. Level 3 Investments MillionsLevel 1InvestmentsLevel 2InvestmentsFixedMaturitiesCommonequitysecuritiesConvertiblefixedmaturitiesOther long-terminvestments Total Balance at January 1, 2011$1,894.4$5,477.4$128.4$71.2$—$330.2(1) $7,901.6(1) Total realized and unrealized gains (losses)(50.5)115.9(10.6)(4.9)—22.5 72.4 Foreign currency gains (losses) through OCI.3(84.4)(4.4)1.6—(5.0) (91.9) Amortization/Accretion2.4(40.7)(.1)——— (38.4) Purchases6,690.73,987.4212.819.7—27.9 10,938.5 Sales(6,782.1)(4,596.9)1.6(55.5)—(83.4) (11,516.3) Transfers in—111.81.0——— 112.8 Transfers out—(1.0)(111.8)——— (112.8) Balance at September 30, 2011$1,755.2$4,969.5$216.9$32.1$—$292.2(1) $7,265.9(1) (1) Excludes carrying value o

[3] September 30, 2012MillionsFair value Level 1 Inputs Level 2 Inputs Level 3 InputsFixed maturity investments: US Government and agency obligations$394.7 $340.5 $54.2 $— Debt securities issued by corporations: Consumer597.2 — 597.2 —Industrial488.1 — 488.1 —Financials405.4 1.1 404.3 —Communications217.1 — 217.1 —Basic materials175.1 — 175.1 —Energy172.9 — 172.9 —Utilities204.4 — 204.4 —Technology21.1 — 21.1 —Diversified— — — —Total debt securities issued by corporations:2,281.3 1.1 2,280.2 — Municipal obligations3.8 — 3.8 —Mortgage-backed and asset-backed securities2,047.4 — 2,041.9 5.5Foreign government, agency and provincial obligations476.4 56.9 419.5 —Preferred stocks86.1 — 15.5 70.6Total fixed maturity investments (1)5,289.7 398.5 4,815.1 76.1 Short-term investments917.3 917.3 — — Common equity securities: Financials316.1 277.9 2.3 35.9Consumer249.7 249.6 .1 —Basic materials118.2 118.2 — —Energy96.8 96.8 — —Technology51.6 51.6 — —Utilities42.9 42.7 .2 —Other129.5 73.2 56.3 —Total common equity securities1,004.8 910.0 58.9 35.9 Convertible fixed maturity investments142.0 — 142.0 —Other long-term investments (2) 272.6 — — 272.6Total investments$7,626.4 $2,225.8 $5,016.0 $384.6 (1) Carrying value includes $377.3 that is classified as assets held for sale relating to discontinued operations. (2) Excludes carrying value of $34.3 associated with other long-term investment limited partnerships accounted for using the equity method and $0.3 related to forward contrac

[4]