Losses and Loss Expenses - Narrative (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ (62,728)	$ (142,042)
Insurance Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(21,869)	(44,813)
Insurance Segment | Professional Malpractice Liability Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(56,390)	1,500
Insurance Segment | Casualty Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(75,674)	(45,665)
Insurance Segment | Discontinued Casualty [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(50,400)
Insurance Segment | Property Insurance Product Line [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	93,536	8,984
Insurance Segment | Property London wholesale [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	49,300
Insurance Segment | Energy Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	38,200
Insurance Segment | Specialty Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	16,659	(9,632)
Reinsurance Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(40,859)	(97,229)
Reinsurance Segment | Casualty And Other Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	69,166	(58,808)
Reinsurance Segment | Casualty Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ 64,000	(52,100)
Reinsurance Segment | Casualty Lines [Member] | Maximum
Liability for Claims and Claims Adjustment Expense [Line Items]
Discount rate	2.50%
Reinsurance Segment | Casualty Lines [Member] | 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	$ 5,200	(6,700)
Reinsurance Segment | Property And Other Short Tail Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(110,025)	(38,421)
Reinsurance Segment | Property Catastrophe Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(43,100)	(23,300)
Reinsurance Segment | Property Other Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(63,400)	(3,300)
Reinsurance Segment | Marine And Aviation Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ (3,500)	$ (11,800)