Unpaid Losses And Loss Adjustment Expenses (Reconciliation Of Consolidated Reserves For Losses And Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 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	$ 158,360	$ (56,741)	158,360	(56,741)
Adjusted net reserves for losses and loss adjustment expenses, beginning of year			8,267,077	8,178,547
Incurred losses and loss adjustment expenses, current accident year	1,226,540	660,122	2,549,471	1,903,645
Incurred losses and loss adjustment expenses, prior accident years			(335,494)	(327,064)
Total incurred losses and loss adjustment expenses			2,213,977	1,576,581
Payments, current accident year			342,055	319,049
Payments, prior accident years			1,185,689	1,219,755
Total payments			1,527,744	1,538,804
Effect of foreign currency rate changes			10,582	38
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	8,976,594	8,216,362	8,976,594	8,216,362
Reinsurance recoverable on unpaid losses	2,466,554	2,041,928	2,466,554	2,041,928	$ 2,006,945
Gross reserves for losses and loss adjustment expenses, end of period	$ 11,443,148	$ 10,258,290	$ 11,443,148	$ 10,258,290