Investment Securities (Fair Value Measurement 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
Fair Value
Investment securities
Fair value investments	6,565,500,000	[1],[2]	6,571,900,000	[3],[4]	6,950,700,000	[1],[2]	7,500,800,000	[3],[4]
Level 1 Inputs
Investment securities
Fair value investments	1,420,600,000	[5]	1,151,500,000	[6]	1,355,100,000	[5]	1,033,100,000	[6]
Level 2 Inputs
Investment securities
Fair value investments	4,721,200,000		5,006,400,000		5,206,100,000		6,088,200,000
Debt securities issued by corporations
Investment securities
Fair value investments	2,314,800,000	[7]			2,385,100,000	[7]
Mortgage-backed and asset-backed securities | Fair Value
Investment securities
Fair value investments	1,834,600,000				2,095,600,000
Mortgage-backed and asset-backed securities | Level 2 Inputs
Investment securities
Fair value investments	1,788,600,000				2,073,500,000
Mortgage-backed and asset-backed securities | Level 3 Inputs
Investment securities
Fair value investments	46,000,000				22,100,000
Fixed maturity investments. | Level 3 Inputs
Investment securities
Fair value investments	116,900,000		119,700,000		92,900,000		78,900,000
Common equity securities | Level 3 Inputs
Investment securities
Fair value investments	37,600,000		35,800,000		37,300,000		32,300,000
Convertible fixed maturity investments | Level 3 Inputs
Investment securities
Fair value investments	0		0		0		0
Other long-term investments | Level 3 Inputs
Investment securities
Fair value investments	269,200,000		258,500,000	[4]	259,300,000		268,300,000	[3],[4]
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 | 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 | U.S. Government and agency obligations | Fair Value
Investment securities
Fair value investments	512,300,000				440,100,000
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1 Inputs
Investment securities
Fair value investments	445,500,000				369,100,000
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2 Inputs
Investment securities
Fair value investments	66,800,000				71,000,000
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Fair Value
Investment securities
Fair value investments	2,314,800,000				2,385,100,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 Inputs
Investment securities
Fair value investments	0				1,000,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 Inputs
Investment securities
Fair value investments	2,314,800,000				2,384,100,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Fair Value
Investment securities
Fair value investments	705,400,000				727,100,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 1 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 2 Inputs
Investment securities
Fair value investments	705,400,000				727,100,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Fair Value
Investment securities
Fair value investments	268,400,000				330,800,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 1 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 2 Inputs
Investment securities
Fair value investments	268,400,000				330,800,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Fair Value
Investment securities
Fair value investments	393,400,000				401,400,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 1 Inputs
Investment securities
Fair value investments	0				1,000,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 2 Inputs
Investment securities
Fair value investments	393,400,000				400,400,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Fair Value
Investment securities
Fair value investments	287,400,000				276,100,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 1 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 2 Inputs
Investment securities
Fair value investments	287,400,000				276,100,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Basic materials | Fair Value
Investment securities
Fair value investments	157,900,000				189,100,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Basic materials | Level 1 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Basic materials | Level 2 Inputs
Investment securities
Fair value investments	157,900,000				189,100,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Basic materials | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Fair Value
Investment securities
Fair value investments	213,500,000				181,500,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 1 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 2 Inputs
Investment securities
Fair value investments	213,500,000				181,500,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Fair Value
Investment securities
Fair value investments	188,700,000				204,200,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 1 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 2 Inputs
Investment securities
Fair value investments	188,700,000				204,200,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Fair Value
Investment securities
Fair value investments	88,200,000				54,000,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 1 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 2 Inputs
Investment securities
Fair value investments	88,200,000				54,000,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Fair Value
Investment securities
Fair value investments	11,900,000				20,900,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Level 1 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Level 2 Inputs
Investment securities
Fair value investments	11,900,000				20,900,000
Fair value measured on a recurring basis | Debt securities issued by corporations | Other: | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Municipal obligations | Fair Value
Investment securities
Fair value investments	5,000,000				5,200,000
Fair value measured on a recurring basis | Municipal obligations | Level 1 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Municipal obligations | Level 2 Inputs
Investment securities
Fair value investments	5,000,000				5,200,000
Fair value measured on a recurring basis | Municipal obligations | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Fair Value
Investment securities
Fair value investments	1,834,600,000				2,095,600,000
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 1 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 2 Inputs
Investment securities
Fair value investments	1,788,600,000				2,073,500,000
Fair value measured on a recurring basis | Mortgage-backed and asset-backed securities | Level 3 Inputs
Investment securities
Fair value investments	46,000,000				22,100,000
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Fair Value
Investment securities
Fair value investments	439,700,000				521,900,000
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 1 Inputs
Investment securities
Fair value investments	46,700,000				52,100,000
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 2 Inputs
Investment securities
Fair value investments	393,000,000				469,800,000
Fair value measured on a recurring basis | Foreign government, agency and provincial obligations | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Non-cumulative perpetual preference shares | Fair Value
Investment securities
Fair value investments	86,300,000				86,400,000
Fair value measured on a recurring basis | Non-cumulative perpetual preference shares | Level 1 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Non-cumulative perpetual preference shares | Level 2 Inputs
Investment securities
Fair value investments	15,400,000				15,600,000
Fair value measured on a recurring basis | Non-cumulative perpetual preference shares | Level 3 Inputs
Investment securities
Fair value investments	70,900,000				70,800,000
Fair value measured on a recurring basis | Fixed maturity investments. | Fair Value
Investment securities
Fair value investments	5,192,700,000	[1]			5,534,300,000	[8]
Fair value measured on a recurring basis | Fixed maturity investments. | Level 1 Inputs
Investment securities
Fair value investments	492,200,000	[1]			422,200,000	[8]
Fair value measured on a recurring basis | Fixed maturity investments. | Level 2 Inputs
Investment securities
Fair value investments	4,583,600,000	[1]			5,019,200,000	[8]
Fair value measured on a recurring basis | Fixed maturity investments. | Level 3 Inputs
Investment securities
Fair value investments	116,900,000	[1]			92,900,000	[8]
Fair value measured on a recurring basis | Short-term investments | Fair Value
Investment securities
Fair value investments	606,800,000				630,600,000
Fair value measured on a recurring basis | Short-term investments | Level 1 Inputs
Investment securities
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 | Common equity securities | Consumer | Fair Value
Investment securities
Fair value investments	289,400,000				255,600,000
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 1 Inputs
Investment securities
Fair value investments	289,400,000				255,600,000
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 2 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Common equity securities | Consumer | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Common equity securities | Industrial | Fair Value
Investment securities
Fair value investments	69,100,000				41,900,000
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 1 Inputs
Investment securities
Fair value investments	69,100,000				41,900,000
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 2 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Common equity securities | Industrial | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Common equity securities | Financials | Fair Value
Investment securities
Fair value investments	301,100,000				324,500,000
Fair value measured on a recurring basis | Common equity securities | Financials | Level 1 Inputs
Investment securities
Fair value investments	263,500,000				286,300,000
Fair value measured on a recurring basis | Common equity securities | Financials | Level 2 Inputs
Investment securities
Fair value investments	0				900,000
Fair value measured on a recurring basis | Common equity securities | Financials | Level 3 Inputs
Investment securities
Fair value investments	37,600,000				37,300,000
Fair value measured on a recurring basis | Common equity securities | Communications | Fair Value
Investment securities
Fair value investments	45,700,000				35,200,000
Fair value measured on a recurring basis | Common equity securities | Communications | Level 1 Inputs
Investment securities
Fair value investments	45,700,000				35,200,000
Fair value measured on a recurring basis | Common equity securities | Communications | Level 2 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Common equity securities | Communications | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Common equity securities | Basic materials | Fair Value
Investment securities
Fair value investments	62,200,000				103,300,000
Fair value measured on a recurring basis | Common equity securities | Basic materials | Level 1 Inputs
Investment securities
Fair value investments	62,200,000				103,300,000
Fair value measured on a recurring basis | Common equity securities | Basic materials | Level 2 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Common equity securities | Basic materials | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Common equity securities | Energy | Fair Value
Investment securities
Fair value investments	75,700,000				101,000,000
Fair value measured on a recurring basis | Common equity securities | Energy | Level 1 Inputs
Investment securities
Fair value investments	75,700,000				101,000,000
Fair value measured on a recurring basis | Common equity securities | Energy | Level 2 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Common equity securities | Energy | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Common equity securities | Utilities | Fair Value
Investment securities
Fair value investments	36,700,000				43,600,000
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 1 Inputs
Investment securities
Fair value investments	36,500,000				43,400,000
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 2 Inputs
Investment securities
Fair value investments	200,000				200,000
Fair value measured on a recurring basis | Common equity securities | Utilities | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Common equity securities | Technology | Fair Value
Investment securities
Fair value investments	72,700,000				55,000,000
Fair value measured on a recurring basis | Common equity securities | Technology | Level 1 Inputs
Investment securities
Fair value investments	72,700,000				55,000,000
Fair value measured on a recurring basis | Common equity securities | Technology | Level 2 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Common equity securities | Technology | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Common equity securities | Other: | Fair Value
Investment securities
Fair value investments	76,900,000				69,600,000
Fair value measured on a recurring basis | Common equity securities | Other: | Level 1 Inputs
Investment securities
Fair value investments	13,600,000				11,200,000
Fair value measured on a recurring basis | Common equity securities | Other: | Level 2 Inputs
Investment securities
Fair value investments	63,300,000				58,400,000
Fair value measured on a recurring basis | Common equity securities | Other: | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Fair Value
Investment securities
Fair value investments	1,029,500,000				1,029,700,000
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 1 Inputs
Investment securities
Fair value investments	928,400,000				932,900,000
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 2 Inputs
Investment securities
Fair value investments	63,500,000				59,500,000
Fair value measured on a recurring basis | Common equity securities | Common equity securities | Level 3 Inputs
Investment securities
Fair value investments	37,600,000				37,300,000
Fair value measured on a recurring basis | Convertible fixed maturity investments | Fair Value
Investment securities
Fair value investments	74,100,000				127,400,000
Fair value measured on a recurring basis | Convertible fixed maturity investments | Level 1 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Convertible fixed maturity investments | Level 2 Inputs
Investment securities
Fair value investments	74,100,000				127,400,000
Fair value measured on a recurring basis | Convertible fixed maturity investments | Level 3 Inputs
Investment securities
Fair value investments	0				0
Fair value measured on a recurring basis | Other long-term investments | Fair Value
Investment securities
Fair value investments	269,200,000	[2]			259,300,000	[9]
Fair value measured on a recurring basis | Other long-term investments | Level 1 Inputs
Investment securities
Fair value investments	0	[2]			0	[9]
Fair value measured on a recurring basis | Other long-term investments | Level 2 Inputs
Investment securities
Fair value investments	0	[2]			0	[9]
Fair value measured on a recurring basis | Other long-term investments | Level 3 Inputs
Investment securities
Fair value investments	269,200,000	[2]			259,300,000	[9]
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 | Forward Contracts | Level 2 Inputs
Investment securities
Realized and unrealized (losses) gains	(100,000)				(100,000)
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] Carrying value includes $260.8 that is classified as assets held for sale relating to discontinued operations.

[2] 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.

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

[4] 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.

[5] 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.

[6] 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

[7] Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor's Financial Services LLC ("Standard & Poor’s") and 2) Moody’s Investor Service ("Moody's").

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

[9] 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.