Reinsurance (Effect Of Reinsurance And Retrocessional Reinsurance On Premiums Written And Earned) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Effects of Reinsurance [Line Items]
Direct premiums written	$ 1,645,089	$ 1,497,202
Direct premiums earned	1,646,508	1,460,860
Assumed premiums written	609,953	549,682
Assumed premiums earned	303,770	336,942
Ceded premiums written	(777,949)	(712,082)
Ceded premiums earned	(746,301)	(646,781)
Net premiums written	1,477,093	1,334,802
Net premiums earned	1,203,977	1,151,021
Underwriting [Member]
Effects of Reinsurance [Line Items]
Direct premiums written	1,127,388	1,039,378
Direct premiums earned	1,131,556	1,037,935
Assumed premiums written	578,097	546,313
Assumed premiums earned	287,375	335,490
Ceded premiums written	(228,632)	(251,557)
Ceded premiums earned	(215,166)	(222,523)
Net premiums written	1,476,853	1,334,134
Net premiums earned	1,203,765	1,150,902
Program Services & Other [Member]
Effects of Reinsurance [Line Items]
Direct premiums written	517,701	457,824
Direct premiums earned	514,952	422,925
Assumed premiums written	31,856	3,369
Assumed premiums earned	16,395	1,452
Ceded premiums written	(549,317)	(460,525)
Ceded premiums earned	(531,135)	(424,258)
Net premiums written	240	668
Net premiums earned	$ 212	$ 119