Losses and Loss Expenses - Net (favorable) adverse prior year loss development of loss and loss expense reserves by operating segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ (9,136)	$ 24,004
Insurance Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(5,295)	(4,642)
Insurance Segment | Professional
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(1,830)	(5)
Insurance Segment | Casualty
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(5,115)	(4,770)
Insurance Segment | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	1,257	81
Insurance Segment | Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	393	52
Reinsurance Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(3,841)	28,646
Reinsurance Segment | Property and other short-tail lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(18,994)	(54,916)
Reinsurance Segment | Casualty and other long-tail lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ 15,153	$ 83,562