Unpaid Losses And Loss Adjustment Expenses (Reconciliation Of Consolidated Reserves For Losses And Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims [Abstract]
Net reserves for losses and loss adjustment expenses, beginning of year			$ 8,108,717	$ 8,235,288
Foreign currency movements	$ 57,991	$ (42,388)	57,991	(42,388)
Adjusted net reserves for losses and loss adjustment expenses, beginning of year			8,166,708	8,192,900
Incurred losses and loss adjustment expenses, current accident year	684,585	651,238	1,322,931	1,243,523
Incurred losses and loss adjustment expenses, prior accident years			(184,367)	(246,314)
Total incurred losses and loss adjustment expenses			1,138,564	997,209
Payments, current accident year			186,138	155,573
Payments, prior accident years			829,126	874,698
Total payments			1,015,264	1,030,271
Effect of foreign currency rate changes			2,333	1,374
Net reserves for losses and loss adjustment expenses of acquired insurance companies			12,702	0
Net reserves for losses and loss adjustment expenses, end of period	8,305,043	8,161,212	8,305,043	8,161,212
Reinsurance recoverable on unpaid losses	2,007,652	2,038,687	2,007,652	2,038,687	$ 2,006,945
Gross reserves for losses and loss adjustment expenses, end of period	$ 10,312,695	$ 10,199,899	$ 10,312,695	$ 10,199,899