Losses and Loss Expenses (Details) - Prior Year Development - Insurance (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	$ (315,895,000)	$ (284,867,000)	$ (372,862,000)
Insurance Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(140,067,000)	(76,516,000)	(127,411,000)
Insurance Segment [Member] | Property
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(46,733,000)	(8,922,000)	(23,525,000)
Insurance Segment [Member] | Property | Catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(19,600,000)
Insurance Segment [Member] | Property | 2010 Accident Year | Catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(12,500,000)
Insurance Segment [Member] | Property | 2011 Accident Year | Non-catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(26,500,000)
Insurance Segment [Member] | Property | 2011 Accident Year | Catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(6,100,000)
Insurance Segment [Member] | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(61,633,000)	34,500,000	28,987,000
Insurance Segment [Member] | Casualty | 2010 Accident Year | Deepwater Horizon
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross (favorable)adverse prior year loss development		135,600,000
Net (favorable) adverse prior year loss development		33,400,000
Insurance Segment [Member] | International primary casualty | 2006-2011 Accident Years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(25,500,000)
Insurance Segment [Member] | Excess casualty | 2005 and prior Accident Years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(50,700,000)
Insurance Segment [Member] | Environmental lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross (favorable)adverse prior year loss development			15,500,000
Insurance Segment [Member] | Professional Lines [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(106,368,000)	(87,520,000)	(118,552,000)
Insurance Segment [Member] | Professional Lines [Member] | Clash losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development		(88,500,000)
Insurance Segment [Member] | Professional Lines [Member] | 2006 and prior Accident Years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	8,800,000
Insurance Segment [Member] | Core professional businesses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross (favorable)adverse prior year loss development			(216,300,000)
Insurance Segment [Member] | Core professional businesses | Clash losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(120,800,000)
Insurance Segment [Member] | Core professional businesses | Non clash losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	1,500,000
Insurance Segment [Member] | Core professional businesses | 2007 Accident Year | Clash losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(48,400,000)
Insurance Segment [Member] | Core professional businesses | 2010 Accident Year | Clash losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(44,200,000)
Insurance Segment [Member] | Core professional businesses | 2011 Accident Year | Clash losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(37,000,000)
Insurance Segment [Member] | Design portfolio
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross (favorable)adverse prior year loss development			6,200,000
Insurance Segment [Member] | Select portfolio
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross (favorable)adverse prior year loss development			27,400,000
Insurance Segment [Member] | Specialty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(61,761,000)	(71,285,000)	(66,278,000)
Insurance Segment [Member] | Aerospace | Non-catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(44,000,000)
Insurance Segment [Member] | Specie | Non-catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(4,500,000)
Insurance Segment [Member] | Product recall | Non-catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(4,000,000)
Insurance Segment [Member] | London Markets
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(8,600,000)
Insurance Segment [Member] | Other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	136,428,000	56,711,000	51,957,000
Insurance Segment [Member] | Other | Claims audit
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	11,800,000
Insurance Segment [Member] | Other | Recent settlements
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	44,900,000
Insurance Segment [Member] | Other | 2006-2011 Accident Years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	80,400,000
Insurance Segment [Member] | Apartments | 2006-2011 Accident Years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	36,500,000
Insurance Segment [Member] | New York contractors | 2006-2011 Accident Years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	38,400,000
Insurance Segment [Member] | International | Core professional businesses | Non clash losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	39,500,000
Insurance Segment [Member] | United States | Casualty | 2006-2011 Accident Years
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	15,700,000
Insurance Segment [Member] | United States | Risk management lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross (favorable)adverse prior year loss development			45,100,000
Insurance Segment [Member] | United States | Core professional businesses | Non clash losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(18,200,000)
Insurance Segment [Member] | United States | Design portfolio
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(9,600,000)
Insurance Segment [Member] | United States | Private commercial lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross (favorable)adverse prior year loss development			63,700,000
Insurance Segment [Member] | Bermuda | Core professional businesses | Non clash losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	(19,800,000)
Insurance Segment [Member] | Canada | Design portfolio
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net (favorable) adverse prior year loss development	22,900,000
Insurance Segment [Member] | Switzerland | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross (favorable)adverse prior year loss development			$ (26,000,000)