Reinsurance (Effect Of Reinsurance And Retrocessional Reinsurance On Premiums Written And Earned) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Effects of Reinsurance [Line Items]
Direct premiums written	$ 1,942,439	$ 1,737,563	$ 5,563,372	$ 4,967,078
Direct premiums earned	1,795,679	1,594,457	5,139,862	4,596,389
Assumed premiums written	322,053	306,208	1,203,309	1,121,010
Assumed premiums earned	347,830	326,424	996,320	982,370
Ceded premiums written	(894,108)	(827,500)	(2,613,566)	(2,356,409)
Ceded premiums earned	(843,477)	(735,558)	(2,432,712)	(2,094,231)
Net premiums written	1,370,384	1,216,271	4,153,115	3,731,679
Net premiums earned	1,300,032	1,185,323	3,703,470	3,484,528
Underwriting [Member]
Effects of Reinsurance [Line Items]
Direct premiums written	1,358,243	1,207,605	3,808,759	3,429,666
Direct premiums earned	1,219,103	1,113,763	3,510,203	3,221,946
Assumed premiums written	282,506	274,898	1,129,440	1,081,171
Assumed premiums earned	327,431	313,378	946,648	965,586
Ceded premiums written	(271,094)	(266,788)	(786,704)	(781,126)
Ceded premiums earned	(246,965)	(242,120)	(754,505)	(703,785)
Net premiums written	1,369,655	1,215,715	4,151,495	3,729,711
Net premiums earned	1,299,569	1,185,021	3,702,346	3,483,747
Program services and other [Member]
Effects of Reinsurance [Line Items]
Direct premiums written	584,196	529,958	1,754,613	1,537,412
Direct premiums earned	576,576	480,694	1,629,659	1,374,443
Assumed premiums written	39,547	31,310	73,869	39,839
Assumed premiums earned	20,399	13,046	49,672	16,784
Ceded premiums written	(623,014)	(560,712)	(1,826,862)	(1,575,283)
Ceded premiums earned	(596,512)	(493,438)	(1,678,207)	(1,390,446)
Net premiums written	729	556	1,620	1,968
Net premiums earned	$ 463	$ 302	$ 1,124	$ 781