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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Premiums written
Direct									$ 208,282	$ 130,681	$ 76,511
Assumed									2,166,294	1,880,629	1,474,061
Ceded									(839,264)	(595,127)	(482,336)
Net premiums written	$ 219,499	$ 284,222	$ 519,916	$ 511,675	$ 236,651	$ 266,820	$ 508,677	$ 404,035	1,535,312	1,416,183	1,068,236
Premiums earned
Direct									157,112	98,182	66,027
Assumed									1,874,993	1,769,088	1,450,047
Ceded									(628,675)	(466,719)	(453,658)
Net premiums earned	351,901	346,521	351,402	353,606	361,575	362,388	379,828	296,760	1,403,430	1,400,551	1,062,416
Claims and claim expenses
Gross claims and claim expenses incurred									710,651	544,972	228,581
Claims and claim expenses recovered									(179,820)	(96,734)	(30,634)
Net claims and claim expenses incurred	$ 123,901	$ 112,575	$ 167,750	$ 126,605	$ 102,013	$ 100,028	$ 169,344	$ 76,853	$ 530,831	$ 448,238	$ 197,947