(Rollforward of Fair Value Measurements by Level) (Details) (USD $)	6 Months Ended				12 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Dec. 31, 2012		Dec. 31, 2011
Investment securities
Fixed maturity investments reclassified to assets held for sale					$ (338,100,000)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	8.0		21.0
Level 1 Inputs
Investment securities
Fair value investments	1,355,100,000	[1]	1,033,100,000	[2]	1,033,100,000	[2]
Total realized and unrealized gains (losses)	89,900,000	[1],[3]	3,000,000	[2],[4]
Foreign currency gains (losses) through OCI	(5,600,000)	[1]	300,000	[2]
Amortization/Accretion	(600,000)	[1]	(200,000)	[2]
Purchases	549,000,000	[1]	596,300,000	[2]
Sales	(583,400,000)	[1]	(481,400,000)	[2]
Net change in investments related to purchases and sales of consolidated affiliates	14,500,000	[1]
Transfers in	1,700,000	[1]	500,000	[2]
Transfers out	0	[1]	(100,000)	[2]
Fair value investments	1,420,600,000	[1]	1,151,500,000	[2]
Level 2 Inputs
Investment securities
Fair value investments	5,206,100,000		6,088,200,000		6,088,200,000
Total realized and unrealized gains (losses)	(55,300,000)	[3]	44,800,000	[4]
Foreign currency gains (losses) through OCI	(44,200,000)		(10,200,000)
Amortization/Accretion	(27,000,000)		(23,300,000)
Purchases	2,345,500,000		2,757,600,000
Sales	(2,713,900,000)		(3,871,300,000)
Net change in investments related to purchases and sales of consolidated affiliates	2,700,000
Transfers in	8,000,000		21,100,000
Transfers out	(700,000)		(500,000)
Fair value investments	4,721,200,000		5,006,400,000
Fair Value
Investment securities
Fair value investments	6,950,700,000	[5],[6]	7,500,800,000	[7],[8]	7,500,800,000	[7],[8]
Total realized and unrealized gains (losses)	49,300,000	[3]	52,900,000	[4]
Foreign currency gains (losses) through OCI	(51,300,000)		(10,500,000)
Amortization/Accretion	(27,600,000)		(24,300,000)
Purchases	2,963,200,000		3,515,000,000
Sales	(3,336,000,000)		(4,462,000,000)
Net change in investments related to purchases and sales of consolidated affiliates	17,200,000
Transfers in	9,700,000		21,600,000
Transfers out	(9,700,000)		(21,600,000)
Fair value investments	6,565,500,000	[5],[6]	6,571,900,000	[7],[8]
Fixed maturity investments. | Level 3 Inputs
Investment securities
Fair value investments	92,900,000		78,900,000		78,900,000
Total realized and unrealized gains (losses)	(300,000)	[3]	7,400,000	[4]
Foreign currency gains (losses) through OCI	0		700,000
Amortization/Accretion	0		(800,000)
Purchases	32,300,000		138,800,000
Sales	0		(84,300,000)
Net change in investments related to purchases and sales of consolidated affiliates	0
Transfers in	0		0
Transfers out	(8,000,000)		(21,000,000)
Fair value investments	116,900,000		119,700,000
Common equity securities | Level 3 Inputs
Investment securities
Fair value investments	37,300,000		32,300,000		32,300,000
Total realized and unrealized gains (losses)	800,000	[3]	1,000,000	[4]
Foreign currency gains (losses) through OCI	0		0
Amortization/Accretion	0		0
Purchases	500,000		2,500,000
Sales	0		0
Net change in investments related to purchases and sales of consolidated affiliates	0
Transfers in	0		0
Transfers out	(1,000,000)		0
Fair value investments	37,600,000		35,800,000
Convertible Debt Securities | Level 3 Inputs
Investment securities
Fair value investments	0		0		0
Total realized and unrealized gains (losses)	0	[3]	0	[4]
Foreign currency gains (losses) through OCI	0		0
Amortization/Accretion	0		0
Purchases	0		0
Sales	0		0
Net change in investments related to purchases and sales of consolidated affiliates	0
Transfers in	0		0
Transfers out	0		0
Fair value investments	0		0
Other long-term investments | Level 3 Inputs
Investment securities
Fair value investments	259,300,000		268,300,000	[7],[8]	268,300,000	[7],[8]
Total realized and unrealized gains (losses)	14,200,000	[3]	(3,300,000)	[4]
Foreign currency gains (losses) through OCI	(1,500,000)		(1,300,000)
Amortization/Accretion	0
Purchases	35,900,000		19,800,000
Sales	(38,700,000)		(25,000,000)
Net change in investments related to purchases and sales of consolidated affiliates	0
Transfers in	0		0
Transfers out	0		0
Fair value investments	269,200,000		258,500,000	[8]
Fair value measured on a recurring basis | Level 1 Inputs
Investment securities
Fair value investments	2,002,700,000				1,985,700,000
Fair value measured on a recurring basis | Level 2 Inputs
Investment securities
Fair value investments	4,745,900,000				5,206,100,000
Fair value measured on a recurring basis | Level 3 Inputs
Investment securities
Fair value investments	423,700,000				389,500,000
Fair value measured on a recurring basis | Fair Value
Investment securities
Fair value investments	7,172,300,000				7,581,300,000
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs
Investment securities
Fair value investments	492,200,000	[5]			422,200,000	[9]
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs
Investment securities
Fair value investments	4,583,600,000	[5]			5,019,200,000	[9]
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs
Investment securities
Fair value investments	116,900,000	[5]			92,900,000	[9]
Fair value measured on a recurring basis | Fixed maturity investments. | Fair Value
Investment securities
Fair value investments	5,192,700,000	[5]			5,534,300,000	[9]
Fair value measured on a recurring basis | Convertible Debt Securities | Level 1 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Convertible Debt Securities | Level 2 Inputs
Investment securities
Fair value investments	74,100,000				127,400,000
Fair value measured on a recurring basis | Convertible Debt Securities | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Convertible Debt Securities | Fair Value
Investment securities
Fair value investments	74,100,000				127,400,000
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Investment securities
Fair value investments	0	[6]			0	[10]
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Investment securities
Fair value investments	0	[6]			0	[10]
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Investment securities
Fair value investments	269,200,000	[6]			259,300,000	[10]
Carrying value of investment accounted for using the equity method	34,200,000		33,000,000		35,000,000		34,000,000
Fair value measured on a recurring basis | Other long-term investments | Fair Value
Investment securities
Fair value investments	269,200,000	[6]			259,300,000	[10]
Fair value measured on a recurring basis | Forward Contracts | Level 2 Inputs
Investment securities
Realized and unrealized (losses) gains	(100,000)				(100,000)
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Investment securities
Fair value investments	630,600,000
Fair value investments	582,100,000				630,600,000
Carrying value of investment accounted for using the equity method	606,800,000		618,400,000		630,600,000		846,000,000
Realized and unrealized gain	0		500,000		100,000
Fair value measured on a recurring basis | Short-term investments | Level 2 Inputs
Investment securities
Fair value investments	24,700,000				0
Fair value measured on a recurring basis | Short-term investments | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Short-term investments | Fair Value
Investment securities
Fair value investments	606,800,000				630,600,000
Segment, Discontinued Operations
Investment securities
Fixed maturity investments reclassified to assets held for sale	(260,800,000)		111,800,000		338,100,000
Prospector Offshore Fund Ltd [Member] | Fair value measured on a recurring basis | Short-term investments
Investment securities
Realized and unrealized gain	$ 0		$ 0

[1] Excludes carrying value of $630.6 and $606.8 at January 1, 2013 and June 30, 2013 and realized and unrealized gain for the periods of $0.1 and $0.8 associated with short-term investments classified.

[2] Excludes carrying value of $846.0 and $618.4 January 1, 2012 and June 30, 2012 realized and unrealized loss for the period of $0.5 associated with short-term investment

[3] ) Excludes $9.9 realized and unrealized losses associated with the Prospector Funds consolidation of investment-related liabilities.

[4] Excludes $4.2 realized and unrealized losses associated with the Prospector Funds consolidation of investment-related liabilities.

[5] Carrying value includes $260.8 that is classified as assets held for sale relating to discontinued operations.

[6] Excludes carrying value of $34.2 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) related to forward contracts.

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

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

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

[10] 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.