(Rollforward of Fair Value Measurements by Level) (Details) (USD $) In Millions, unless otherwise specified		3 Months Ended																																																																					3 Months Ended	12 Months Ended	3 Months Ended
	Dec. 31, 2012	Mar. 31, 2013 Level 1 Inputs		Mar. 31, 2012 Level 1 Inputs		Mar. 31, 2013 Level 2 Inputs	Mar. 31, 2012 Level 2 Inputs	Mar. 31, 2013 Fair Value		Mar. 31, 2012 Fair Value		Mar. 31, 2013 Fixed maturity investments. Level 3 Inputs	Mar. 31, 2012 Fixed maturity investments. Level 3 Inputs	Mar. 31, 2013 Common equity securities Level 3 Inputs	Mar. 31, 2012 Common equity securities Level 3 Inputs	Mar. 31, 2013 Convertible Debt Securities Level 3 Inputs	Mar. 31, 2012 Convertible Debt Securities Level 3 Inputs	Mar. 31, 2013 Other long-term investments Level 3 Inputs	Mar. 31, 2012 Other long-term investments Level 3 Inputs		Mar. 31, 2013 Fair value measured on a recurring basis Level 1 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Level 1 Inputs	Mar. 31, 2013 Fair value measured on a recurring basis Level 2 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Level 2 Inputs	Mar. 31, 2013 Fair value measured on a recurring basis Level 3 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Level 3 Inputs	Mar. 31, 2013 Fair value measured on a recurring basis Fair Value	Dec. 31, 2012 Fair value measured on a recurring basis Fair Value	Mar. 31, 2013 Fair value measured on a recurring basis Fixed maturity investments. Level 1 Inputs		Dec. 31, 2012 Fair value measured on a recurring basis Fixed maturity investments. Level 1 Inputs		Mar. 31, 2013 Fair value measured on a recurring basis Fixed maturity investments. Level 2 Inputs		Dec. 31, 2012 Fair value measured on a recurring basis Fixed maturity investments. Level 2 Inputs		Mar. 31, 2013 Fair value measured on a recurring basis Fixed maturity investments. Level 3 Inputs		Dec. 31, 2012 Fair value measured on a recurring basis Fixed maturity investments. Level 3 Inputs		Mar. 31, 2013 Fair value measured on a recurring basis Fixed maturity investments. Fair Value		Dec. 31, 2012 Fair value measured on a recurring basis Fixed maturity investments. Fair Value		Mar. 31, 2013 Fair value measured on a recurring basis Convertible Debt Securities Level 1 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Convertible Debt Securities Level 1 Inputs	Mar. 31, 2013 Fair value measured on a recurring basis Convertible Debt Securities Level 2 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Convertible Debt Securities Level 2 Inputs	Mar. 31, 2013 Fair value measured on a recurring basis Convertible Debt Securities Level 3 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Convertible Debt Securities Level 3 Inputs	Mar. 31, 2013 Fair value measured on a recurring basis Convertible Debt Securities Fair Value	Dec. 31, 2012 Fair value measured on a recurring basis Convertible Debt Securities Fair Value	Mar. 31, 2013 Fair value measured on a recurring basis Other long-term investments Level 1 Inputs		Dec. 31, 2012 Fair value measured on a recurring basis Other long-term investments Level 1 Inputs		Mar. 31, 2013 Fair value measured on a recurring basis Other long-term investments Level 2 Inputs		Dec. 31, 2012 Fair value measured on a recurring basis Other long-term investments Level 2 Inputs		Mar. 31, 2013 Fair value measured on a recurring basis Other long-term investments Level 3 Inputs		Dec. 31, 2012 Fair value measured on a recurring basis Other long-term investments Level 3 Inputs		Mar. 31, 2012 Fair value measured on a recurring basis Other long-term investments Level 3 Inputs	Dec. 31, 2011 Fair value measured on a recurring basis Other long-term investments Level 3 Inputs	Mar. 31, 2013 Fair value measured on a recurring basis Other long-term investments Fair Value		Dec. 31, 2012 Fair value measured on a recurring basis Other long-term investments Fair Value		Mar. 31, 2013 Fair value measured on a recurring basis Forward Contracts Level 2 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Forward Contracts Level 2 Inputs	Mar. 31, 2013 Fair value measured on a recurring basis Short-term investments Level 1 Inputs	Mar. 31, 2012 Fair value measured on a recurring basis Short-term investments Level 1 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Short-term investments Level 1 Inputs	Dec. 31, 2011 Fair value measured on a recurring basis Short-term investments Level 1 Inputs	Mar. 31, 2013 Fair value measured on a recurring basis Short-term investments Level 2 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Short-term investments Level 2 Inputs	Mar. 31, 2013 Fair value measured on a recurring basis Short-term investments Level 3 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Short-term investments Level 3 Inputs	Mar. 31, 2013 Fair value measured on a recurring basis Short-term investments Fair Value	Dec. 31, 2012 Fair value measured on a recurring basis Short-term investments Fair Value	Mar. 31, 2013 Segment, Discontinued Operations	Dec. 31, 2012 Segment, Discontinued Operations	Mar. 31, 2012 Segment, Discontinued Operations
Investment securities
Fair value investments		$ 1,355.1	[1]	$ 1,033.1	[2]	$ 5,206.1	$ 6,088.2	$ 6,950.7	[3],[4]	$ 7,500.8	[5],[6]	$ 92.9	$ 78.9	$ 37.3	$ 32.3	$ 0	$ 0	$ 259.3	$ 268.3	[5],[6]	$ 2,040.8	$ 1,985.7	$ 4,795.9	$ 5,206.1	$ 398.3	$ 389.5	$ 7,235.0	$ 7,581.3	$ 413.1	[3]	$ 422.2	[7]	$ 4,637.2	[3]	$ 5,019.2	[7]	$ 101.0	[3]	$ 92.9	[7]	$ 5,151.3	[3]	$ 5,534.3	[7]	$ 0	$ 0	$ 95.6	$ 127.4	$ 0	$ 0	$ 95.6	$ 127.4	$ 0	[4]	$ 0	[8]	$ 0	[4]	$ 0	[8]	$ 260.5	[4]	$ 259.3	[8]			$ 260.5	[4]	$ 259.3	[8]			$ 630.6				$ 0	$ 0	$ 0	$ 0	$ 654.4	$ 630.6
Total realized and unrealized gains (losses)		80.1	[1]	34.7	[2]	(10.8)	10.1	75.1		58.9		0.1	7.1	(0.1)	0.5	0	0	5.8	6.5
Foreign currency gains (losses) through OCI		(0.4)	[1]	0.7	[2]	(4.8)	50.4	(5.6)		54.5		0	0.7	0	0	0	0	(0.4)	2.7
Amortization/Accretion		(0.4)	[1]	(0.3)	[2]	(14.0)	(12.5)	(14.4)		(13.9)		0	(1.1)	0	0	0	0	0
Purchases		318.6	[1]	166.6	[2]	1,445.8	1,708.2	1,791.3		1,978.2		8.0	97.7	0	0	0	0	18.9	5.7
Sales		(382.3)	[1]	(129.0)	[2]	(1,828.3)	(2,729.4)	(2,233.7)		(2,872.2)		0	(4.1)	0	0	0	0	(23.1)	(9.7)
Net change in investments related to purchases and sales of consolidated affiliates		14.5	[1]			2.7		17.2				0		0		0		0
Transfers in		1.2	[1]	0	[2]	0	0	1.2		0		0	0	0	0	0	0	0	0
Transfers out		0	[1]	0	[2]	(0.8)	0	(1.2)		0		0	0	(0.4)	0	0	0	0	0
Fair value investments		1,386.4	[1]	1,105.8	[2]	4,795.9	5,115.0	6,580.6	[3],[4]	6,706.3	[5],[6]	101.0	179.2	36.8	32.8	0	0	260.5	273.5	[5]	2,040.8	1,985.7	4,795.9	5,206.1	398.3	389.5	7,235.0	7,581.3	413.1	[3]	422.2	[7]	4,637.2	[3]	5,019.2	[7]	101.0	[3]	92.9	[7]	5,151.3	[3]	5,534.3	[7]	0	0	95.6	127.4	0	0	95.6	127.4	0	[4]	0	[8]	0	[4]	0	[8]	260.5	[4]	259.3	[8]			260.5	[4]	259.3	[8]			654.4				0	0	0	0	654.4	630.6
Carrying value of investment accounted for using the equity method																																																													34.9		35.0		33.0	33.2							654.4	877.5	630.6	846.0
Realized and unrealized gain																																																																									0.1	0.3
Realized and unrealized (losses) gains																																																																							(0.2)	(0.1)
Fixed maturity investments reclassified to assets held for sale	$ 338.1																																																																																		$ 296.9	$ 338.1	$ 111.8

[1] Excludes carrying value of $630.6 and $654.4 at January 1, 2013 and March 31, 2013 and realized and unrealized gain for the period of $0.1 associated with short-term investments classified as level 1.

[2] Excludes carrying value of $877.5 and $846.0 at March 31, 2012 and January 1, 2012 and realized and unrealized loss for the period of $0.3 associated with short-term investments classified as level 1.

[3] Carrying value includes $296.9 that is classified as assets held for sale relating to discontinued operations.

[4] Excludes carrying value of $34.9 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.2) related to forward contracts.

[5] Excludes carrying value of $33.0 and $33.2 at March 31, 2012 and January 1, 2012 associated with other long-term investment limited partnerships accounted for using the equity method.

[6] Carrying value includes $111.8 that is classified as assets held for sale relating to discontinued operations.

[7] Carrying value includes $338.1 that is classified as assets held for sale relating to discontinued operations.

[8] Excludes carrying value of $35.0 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) related to currency forward contracts.