Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves	$ 188,949	$ 265,613
Gen Liability Workers Comp And Personal Lines Insurance Segment Aviation Whole Account And Auto Reinsurance Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves	$ (168,100)
Gen Liability Prof Liability Workers Comp And Marine And Energy Insurance Segment Surety And Marine and Energy Reinsurance Segment [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves		$ (211,900)