Reinsurance (Effect Of Reinsurance And Retrocessional Reinsurance On Premiums Written And Earned) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Effects of Reinsurance [Line Items]
Direct premiums written	$ 1,975,844	$ 1,732,313	$ 3,620,933	$ 3,229,515
Direct premiums earned	1,697,675	1,541,072	3,344,183	3,001,932
Assumed premiums written	271,303	265,120	881,256	814,802
Assumed premiums earned	344,720	319,004	648,490	655,946
Ceded premiums written	(941,509)	(816,827)	(1,719,458)	(1,528,909)
Ceded premiums earned	(842,934)	(711,892)	(1,589,235)	(1,358,673)
Net premiums written	1,305,638	1,180,606	2,782,731	2,515,408
Net premiums earned	1,199,461	1,148,184	2,403,438	2,299,205
Underwriting [Member]
Effects of Reinsurance [Line Items]
Direct premiums written	1,323,128	1,182,683	2,450,516	2,222,061
Direct premiums earned	1,159,544	1,070,248	2,291,100	2,108,183
Assumed premiums written	268,837	259,960	846,934	806,273
Assumed premiums earned	331,842	316,718	619,217	652,208
Ceded premiums written	(286,978)	(262,781)	(515,610)	(514,338)
Ceded premiums earned	(292,374)	(239,152)	(507,540)	(461,665)
Net premiums written	1,304,987	1,179,862	2,781,840	2,513,996
Net premiums earned	1,199,012	1,147,814	2,402,777	2,298,726
Program Services & Other [Member]
Effects of Reinsurance [Line Items]
Direct premiums written	652,716	549,630	1,170,417	1,007,454
Direct premiums earned	538,131	470,824	1,053,083	893,749
Assumed premiums written	2,466	5,160	34,322	8,529
Assumed premiums earned	12,878	2,286	29,273	3,738
Ceded premiums written	(654,531)	(554,046)	(1,203,848)	(1,014,571)
Ceded premiums earned	(550,560)	(472,740)	(1,081,695)	(897,008)
Net premiums written	651	744	891	1,412
Net premiums earned	$ 449	$ 370	$ 661	$ 479