Losses and Loss Expenses - Net (favorable) adverse prior year loss development of loss and loss expense reserves by operating segment (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 | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	91,488	(7,356)
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
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	61,000	(50,400)
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 | Casualty and other long-tail lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ 61,452	$ (59,121)