Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves	$ 70,210	$ 108,067
Estimated net losses and loss adjustment expenses	757,970	723,208
2018 Catastrophes [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves	(12,800)
Gen Liability And Workers Comp Insurance Segment Surety Aviation And Whole Account 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	$ (74,600)
Gen Liability Prof Liability Marine And Energy Personal Lines And Workers Comp Insurance Segment Surety And Marine and Energy 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		$ (93,100)