Losses and Loss Expenses - Net losses and loss expenses incurred - Reinsurance (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)
Reinsurance Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(210,083)	(241,600)	(155,314)
Reinsurance Segment | Property and other short-tail lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(134,440)	(173,754)	(85,324)
Reinsurance Segment | Property catastrophe lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(76,600)	(50,300)	12,400
Reinsurance Segment | Property other lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(25,600)	(88,900)
Reinsurance Segment | Marine and aviation lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(32,200)	(34,600)	(34,400)
Reinsurance Segment | Casualty and Other Long-Tail Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(75,643)	(67,846)	(69,990)
Reinsurance Segment | Casualty and other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(65,500)	(40,100)	(44,300)
Reinsurance Segment | Other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	$ (10,000)	$ (27,800)
Reinsurance Segment | Property Other Than Property Catastrophe [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development			(63,400)
Reinsurance Segment | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development			$ (25,700)