Losses and Loss Expenses - Net losses and loss expenses incurred - Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	$ (147,770)	$ (301,542)	$ (306,630)
Insurance Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(41,759)	(91,459)	(65,030)
Reinsurance Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	$ (106,011)	$ (210,083)	$ (241,600)