Reinsurance (Effect Of Reinsurance And Retrocessional Reinsurance On Premiums Written And Earned) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effects of Reinsurance [Line Items]
Direct premiums written	$ 6,584,804	$ 4,172,467	$ 3,560,635
Direct premiums earned	6,235,218	4,068,622	3,506,687
Assumed premiums written	1,279,665	1,334,493	1,236,010
Assumed premiums earned	1,319,613	1,287,395	1,176,205
Ceded premiums written	(3,076,891)	(1,089,173)	(795,625)
Ceded premiums earned	(2,842,771)	(1,108,039)	(817,022)
Net premiums written	4,787,578	4,417,787	4,001,020
Net premiums earned	4,712,060	4,247,978	3,865,870
Underwriting [Member]
Effects of Reinsurance [Line Items]
Direct premiums written	4,562,256	3,919,602	3,560,635
Direct premiums earned	4,384,562	3,777,335	3,506,687
Assumed premiums written	1,236,740	1,333,505	1,236,010
Assumed premiums earned	1,291,032	1,286,043	1,176,205
Ceded premiums written	(1,013,406)	(835,320)	(795,625)
Ceded premiums earned	(964,549)	(815,400)	(817,022)
Net premiums written	4,785,590	4,417,787	4,001,020
Net premiums earned	4,711,045	4,247,978	3,865,870
Program Services [Member]
Effects of Reinsurance [Line Items]
Direct premiums written	2,022,548	252,865	0
Direct premiums earned	1,850,656	291,287	0
Assumed premiums written	42,925	988	0
Assumed premiums earned	28,581	1,352	0
Ceded premiums written	(2,063,485)	(253,853)	0
Ceded premiums earned	(1,878,222)	(292,639)	0
Net premiums written	1,988	0	0
Net premiums earned	$ 1,015	$ 0	$ 0