Reinsurance (Effect Of Reinsurance And Retrocessional Activity On Premiums Written And Earned And On Net Claims And Claim Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums written
Direct									$ 290,730	$ 208,282	$ 130,681
Assumed									2,506,810	2,166,294	1,880,629
Ceded									(926,215)	(839,264)	(595,127)
Net premiums written	$ 288,223	$ 483,221	$ 555,745	$ 544,136	$ 219,499	$ 284,222	$ 519,916	$ 511,675	1,871,325	1,535,312	1,416,183
Premiums earned
Direct									244,285	157,112	98,182
Assumed									2,307,219	1,874,993	1,769,088
Ceded									(833,929)	(628,675)	(466,719)
Net premiums earned (Note 7)	421,473	547,792	382,265	366,045	351,901	346,521	351,402	353,606	1,717,575	1,403,430	1,400,551
Claims and claim expenses
Gross claims and claim expenses incurred									3,420,388	710,651	544,972
Claims and claim expenses recovered									(1,558,960)	(179,820)	(96,734)
Net claims and claim expenses incurred	$ 304,064	$ 1,221,696	$ 142,587	$ 193,081	$ 123,901	$ 112,575	$ 167,750	$ 126,605	$ 1,861,428	$ 530,831	$ 448,238