Losses and Loss Expenses - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ (93,590)	$ (195,647)
Insurance Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(30,680)	(67,498)
Insurance Segment | Professional
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(54,833)	(369)
Insurance Segment | Casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(74,443)	(45,230)
Insurance Segment | Discontinued Casualty [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(50,500)
Insurance Segment | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	91,488	(7,356)
Insurance Segment | Property London wholesale [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	49,400
Insurance Segment | Energy Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	38,300
Insurance Segment | Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	7,108	(14,543)
Reinsurance Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(62,910)	(128,149)
Reinsurance Segment | Casualty and other long-tail lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	61,452	(59,121)
Reinsurance Segment | Casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ 61,000	(50,400)
Reinsurance Segment | Casualty | Maximum
Liability for Claims and Claims Adjustment Expense [Line Items]
Discount rate	2.50%
Reinsurance Segment | Casualty | Minimum
Liability for Claims and Claims Adjustment Expense [Line Items]
Discount rate	(0.75%)
Reinsurance Segment | Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ 500	(8,700)
Reinsurance Segment | Property and other short-tail lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(124,362)	(69,028)
Reinsurance Segment | Property Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(115,000)
Reinsurance Segment | Property Catastrophe Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year		(54,400)
Reinsurance Segment | Property Other Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year		(5,100)
Reinsurance Segment | Marine And Aviation Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ (9,400)	$ (9,600)