(Rollforward of Fair Value Measurements by Level) (Details) (USD $)	3 Months Ended		9 Months Ended			9 Months Ended																																																																														9 Months Ended	12 Months Ended	9 Months Ended													9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Level 1 Inputs		Sep. 30, 2012 Level 1 Inputs		Sep. 30, 2013 Level 2 Inputs		Sep. 30, 2012 Level 2 Inputs		Sep. 30, 2013 Fair Value		Sep. 30, 2012 Fair Value		Sep. 30, 2013 Fixed maturity investments. Level 3 Inputs		Sep. 30, 2012 Fixed maturity investments. Level 3 Inputs		Sep. 30, 2013 Common equity securities Level 3 Inputs		Sep. 30, 2012 Common equity securities Level 3 Inputs		Sep. 30, 2013 Convertible Debt Securities Level 3 Inputs		Sep. 30, 2012 Convertible Debt Securities Level 3 Inputs		Sep. 30, 2013 Other long-term investments Level 3 Inputs		Sep. 30, 2012 Other long-term investments Level 3 Inputs		Sep. 30, 2013 Fair value measured on a recurring basis Level 1 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Level 1 Inputs	Sep. 30, 2013 Fair value measured on a recurring basis Level 2 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Level 2 Inputs	Sep. 30, 2013 Fair value measured on a recurring basis Level 3 Inputs	Dec. 31, 2012 Fair value measured on a recurring basis Level 3 Inputs	Sep. 30, 2013 Fair value measured on a recurring basis Fair Value	Dec. 31, 2012 Fair value measured on a recurring basis Fair Value	Sep. 30, 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		Sep. 30, 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		Sep. 30, 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		Sep. 30, 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		Sep. 30, 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	Sep. 30, 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	Sep. 30, 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	Sep. 30, 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	Sep. 30, 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		Sep. 30, 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		Sep. 30, 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		Sep. 30, 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	Sep. 30, 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		Sep. 30, 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	Sep. 30, 2013 Fair value measured on a recurring basis Short-term investments Level 1 Inputs	Sep. 30, 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	Sep. 30, 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	Sep. 30, 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	Sep. 30, 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	Sep. 30, 2013 Segment, Discontinued Operations	Dec. 31, 2012 Segment, Discontinued Operations	Sep. 30, 2012 Segment, Discontinued Operations	Sep. 30, 2013 Prospector Offshore Fund Ltd [Member] Fair value measured on a recurring basis Short-term investments
Investment securities
Fair value investments						$ 1,355,100,000	[1]	$ 1,033,100,000	[2]	$ 5,206,100,000		$ 6,088,200,000		$ 6,950,700,000	[3],[4]	$ 7,500,800,000	[5],[6]	$ 92,900,000		$ 78,900,000		$ 37,300,000		$ 32,300,000		$ 0		$ 0		$ 259,300,000		$ 268,300,000	[5],[6]	$ 1,999,100,000	$ 1,985,700,000	$ 4,781,300,000	$ 5,206,100,000	$ 490,000,000	$ 389,500,000	$ 7,270,400,000	$ 7,581,300,000	$ 367,300,000	[3]	$ 422,200,000	[7]	$ 4,606,400,000	[3]	$ 5,019,200,000	[7]	$ 180,000,000	[3]	$ 92,900,000	[7]	$ 5,153,700,000	[3]	$ 5,534,300,000	[7]	$ 0	$ 0	$ 84,800,000	$ 127,400,000	$ 0	$ 0	$ 84,800,000	$ 127,400,000	$ 0	[4]	$ 0	[8]	$ 0	[4]	$ 0	[8]	$ 273,200,000	[4]	$ 259,300,000	[8]			$ 273,200,000	[4]	$ 259,300,000	[8]			$ 630,600,000				$ 19,400,000	$ 0	$ 0	$ 0	$ 648,000,000	$ 630,600,000
Total realized and unrealized gains (losses)						140,600,000	[1],[9]	61,700,000	[10],[2]	(70,700,000)	[9]	58,800,000	[10]	80,600,000	[9]	132,100,000	[10]	(3,300,000)	[9]	8,400,000	[10]	0	[9]	11,400,000	[10]	0	[9]	0	[10]	14,000,000	[9]	(8,200,000)	[10]
Foreign currency gains (losses) through OCI						1,400,000	[1]	4,100,000	[2]	17,500,000		70,500,000		18,600,000		78,500,000		500,000		700,000		0		100,000		0		0		(800,000)		3,100,000
Amortization/Accretion						(900,000)	[1]	(600,000)	[2]	(39,700,000)		(34,000,000)		(40,600,000)		(35,300,000)		0		(700,000)		0		0		0		0		0
Purchases						735,900,000	[1]	891,100,000	[2]	3,014,700,000		3,823,700,000		3,875,300,000		4,902,200,000		32,400,000		144,300,000		500,000		2,500,000		0		0		91,800,000		40,600,000
Sales						(877,900,000)	[1]	(680,900,000)	[2]	(3,309,700,000)		(5,048,100,000)		(4,279,400,000)		(5,869,200,000)		(700,000)		(99,200,000)		0		(9,800,000)		0		0		(91,100,000)		(31,200,000)
Net change in investments related to purchases and sales of consolidated affiliates						14,500,000	[1]			2,700,000				17,200,000				0				0				0				0
Transfers in						1,800,000	[1]	0	[2]	32,100,000		56,900,000		124,200,000		56,900,000		90,300,000		0		0		0		0		0		0		0
Transfers out						0	[1]	0	[2]	(91,100,000)		0		(124,200,000)		(56,900,000)		(32,100,000)		(56,300,000)		(1,000,000)		(600,000)		0		0		0		0
Fair value investments	1.3		1.3			1,370,500,000	[1]	1,308,500,000	[2]	4,761,900,000		5,016,000,000		6,622,400,000	[3],[4]	6,709,100,000	[5],[6]	180,000,000		76,100,000		36,800,000		35,900,000		0		0		273,200,000		272,600,000	[6]	1,999,100,000	1,985,700,000	4,781,300,000	5,206,100,000	490,000,000	389,500,000	7,270,400,000	7,581,300,000	367,300,000	[3]	422,200,000	[7]	4,606,400,000	[3]	5,019,200,000	[7]	180,000,000	[3]	92,900,000	[7]	5,153,700,000	[3]	5,534,300,000	[7]	0	0	84,800,000	127,400,000	0	0	84,800,000	127,400,000	0	[4]	0	[8]	0	[4]	0	[8]	273,200,000	[4]	259,300,000	[8]			273,200,000	[4]	259,300,000	[8]			628,600,000				19,400,000	0	0	0	648,000,000	630,600,000
Carrying value of investment accounted for using the equity method																																																																										26,900,000		35,000,000		33,000,000	34,000,000							648,000,000	917,300,000	630,600,000	846,000,000
Realized and unrealized gain	27,000,000	60,400,000	63,600,000	94,900,000																																																																																		0.2	3,900,000
Net Realized and Unrealized Gain (Loss) on Trading Securities	28,200,000	72,700,000	66,100,000	123,200,000																																																																																															14.1
Realized and unrealized (losses) gains																																																																																				(100,000)	(100,000)
Fixed maturity investments reclassified to assets held for sale					338,100,000																																																																																											(239,700,000)	338,100,000	111,800,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3			90.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3			$ 32.1	$ 56.3

[1] Excludes carrying value of $630.6 and $648.0 at January 1, 2013 and September 30, 2013 and realized and unrealized gains for the periods of $0.2 associated with short-term investments

[2] Excludes carrying value of $846.0 and $917.3 at January 1, 2012 and September 30, 2012 and realized and unrealized loss for the period of $3.9 associated with short-term investment

[3] Carrying value includes $239.7 that is classified as assets held for sale relating to discontinued operations.

[4] Excludes carrying value of $26.9 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) related to forward contracts

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

[6] Excludes carrying value of $33.0 and $34.0 at January 1, 2012 and September 30, 2012 associated with other long-term investment limited partnerships accounted for using the equity method.

[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.

[9] ) Excludes $14.1 realized and unrealized losses associated with the Prospector Funds consolidation of investment-related liabilities.

[10]