Losses and Loss Expenses - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ 24,004	$ (43,412)
Insurance Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	(4,600)	(11,358)
Reinsurance Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	28,600	$ (32,054)
Casualty And Other Lines [Member] | Reinsurance Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior year	$ 75,000