Loss And Loss Adjustment Expense Reserves (Activity In The Reserves For Losses And Loss Adjustment Expenses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves at January 1	$ 1,829,412	$ 1,510,613	$ 1,290,248
Reinsurance recoverables on unpaid losses	76,100	180,859	64,001	$ 13,161
Net reserves at January 1	1,648,553	1,446,612	1,277,087
Incurred losses and loss adjustment expense related to:
Current year	2,696,230	2,483,693	2,390,453
Prior year claims and claim adjustment expense	9,794	93,096	54,431
Total incurred losses and loss adjustment expenses	2,706,024	2,576,789	2,444,884
Loss and loss adjustment expense payments related to:
Current year	1,651,550	1,543,828	1,550,789
Prior years	857,872	831,020	724,570
Total payments	2,509,422	2,374,848	2,275,359
Net reserves at December 31	1,845,155	1,648,553	1,446,612
Gross reserves at December 31	$ 1,921,255	$ 1,829,412	$ 1,510,613