Losses and Loss Expenses - Net (favorable) adverse prior year loss development of loss and loss expense reserves by operating segment (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 | Property Insurance Product Line [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	93,536	8,984
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 | Property And Other Short Tail Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(110,025)	(38,421)
Reinsurance Segment | Casualty Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	64,000	(52,100)
Reinsurance Segment | Casualty And Other Lines [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ 69,166	$ (58,808)