Losses and Loss Expenses (Details) - Prior Year Development (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)
Insurance Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(140,067)	(76,516)	(127,411)
Insurance Segment [Member] | Property
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(46,733)	(8,922)	(23,525)
Insurance Segment [Member] | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(61,633)	34,500	28,987
Insurance Segment [Member] | Professional
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(106,368)	(87,520)	(118,552)
Insurance Segment [Member] | Specialty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(61,761)	(71,285)	(66,278)
Insurance Segment [Member] | Other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	136,428	56,711	51,957
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] | 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] | 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] | 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)