Losses and Loss Expenses - Narrative (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ (18,050)	$ (62,728)
Insurance Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	46,588	(21,869)
Reinsurance Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(64,638)	(40,859)
Reinsurance Segment | Property and other short-tail lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(62,883)	(110,025)
Reinsurance Segment | Casualty and other long-tail lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ (1,755)	$ 69,166