Losses and Loss Expenses - Net losses and loss expenses incurred - Reinsurance (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)
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)
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	(140,771)	(134,440)	(173,754)
Reinsurance Segment | Property catastrophe lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(63,000)	(76,600)	(50,300)
Reinsurance Segment | Property other lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(63,100)	(25,600)
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)
Reinsurance Segment | Specialty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	(14,600)
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	34,760	(75,643)	(67,846)
Reinsurance Segment | Casualty and other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	46,300	(65,500)	(40,100)
Reinsurance Segment | Other
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development	$ (11,600)	$ (10,000)
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			(88,900)
Reinsurance Segment | Casualty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year development			$ (27,800)