Losses and Loss Expenses (Details) - Prior Year Development - Reinsurance (USD $) In Thousands, unless otherwise specified	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]
Prior year development	$ (315,895)	$ (284,867)	$ (372,862)
Reinsurance Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(175,828)	(208,351)	(245,451)
Reinsurance Segment [Member] | Property and other short-tail lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(107,613)	(64,267)	(145,768)
Reinsurance Segment [Member] | Property catastrophe lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development		(37,300)	(35,600)
Reinsurance Segment [Member] | Property other lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(49,000)	(40,500)	(87,400)
Reinsurance Segment [Member] | Marine and aviation lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(42,200)	13,500	(22,800)
Reinsurance Segment [Member] | Marine and aviation lines | Large losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(6,100)
Reinsurance Segment [Member] | Casualty and other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(68,215)	(144,084)	(99,683)
Reinsurance Segment [Member] | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development		(109,400)	(25,200)
Reinsurance Segment [Member] | Other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(7,900)
Reinsurance Segment [Member] | Other | Whole account treaties written on Lloyds' Syndicates
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development		(34,700)	(74,500)
Reinsurance Segment [Member] | Structured indemnity
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(8,400)
Reinsurance Segment [Member] | International | Marine and aviation lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(27,300)
Reinsurance Segment [Member] | North America | Marine and aviation lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(4,100)
Reinsurance Segment [Member] | North America | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(60,300)
Reinsurance Segment [Member] | Bermuda | Property catastrophe lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(16,500)
Reinsurance Segment [Member] | Bermuda | Marine and aviation lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	$ (4,900)