Losses and Loss Expenses - Net losses and loss expenses incurred - Insurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	$ (301,542)	$ (306,630)	$ (255,072)
Insurance Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(91,459)	(65,030)	(99,758)
Insurance Segment | Casualty and other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(58,251)	28,720	23,371
Insurance Segment | Excess Casualty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(21,700)
Insurance Segment | EPC
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(38,366)	(6,447)	(57,465)
Insurance Segment | Professional lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	10,813	5,740	17,092
Insurance Segment | Specialty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(5,655)	$ (93,043)	$ (82,756)
Insurance Segment | Marine Lines [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(19,800)
Insurance Segment | Fine Art and Specie [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(5,900)
Insurance Segment | Accident and Health Insurance Product Line [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	26,000
Insurance Segment | International [Member] | Casualty and other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	$ (72,200)