Losses and Loss Expenses - Prior Year Development - Insurance (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	$ (255,076)	$ (289,889)	$ (315,894)
Insurance Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(99,762)	(102,039)	(140,066)
Insurance Segment [Member] | Property
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(57,470)	(46,387)	(46,735)
Insurance Segment [Member] | Property | 2011 Accident Year [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(21,100)
Insurance Segment [Member] | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	38,413	(21,829)	(61,630)
Insurance Segment [Member] | Excess Casualty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(28,600)
Insurance Segment [Member] | Environmental Lines [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	34,900
Insurance Segment [Member] | Lloyds Middle Market [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	33,300
Insurance Segment [Member] | Professional Lines [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	17,094	75,045	(106,360)
Insurance Segment [Member] | Professional Lines [Member] | Clash losses
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development			(120,800)
Insurance Segment [Member] | Specialty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(82,756)	(140,740)	(61,755)
Insurance Segment [Member] | Aerospace [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(61,900)
Insurance Segment [Member] | Marine Lines [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(5,600)
Insurance Segment [Member] | Other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(15,043)	31,872	136,414
Insurance Segment [Member] | United States | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	26,400
Insurance Segment [Member] | United States | Professional Lines [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	40,900
Insurance Segment [Member] | Bermuda | Professional Lines [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(21,800)
Insurance Segment [Member] | International [Member] | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(28,100)
Insurance Segment [Member] | International [Member] | Specialty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	$ (12,200)