Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ceded reserves for losses and loss adjustment expenses on asbestos and environmental policies transferred to a third party by way of a Part VII transfer			$ 69,100
Deferred gain related to the completion of the Part VII transfer	$ 3,900
Favorable development related to the commutation of a property and casualty deposit contract		$ 11,700
Net reserves for losses and loss adjustment expenses of acquired insurance companies	12,702	0
Favorable (adverse) development on prior years' loss reserves	$ 184,367	246,314
Previous Ogden discount rate, percent	2.50%
Current Ogden discount rate, percent	(0.75%)
Gen Liability Personal Lines And Workers Comp US Segment Prof Liability Gen Liability And Marine And Energy Intl Segment Property And Whole Acct Reins 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	$ 195,700
Ogden Rate Change [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves	$ (85,000)
Gen Liability And Workers Comp US Segment Prof Liability And Marine And Energy Intl Segment Property And Workers Comp Reins 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		163,400
Development On Specified Medical And Medical Malpractice [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable (adverse) development on prior years' loss reserves		$ (34,900)