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
	Mar. 31, 2017	Mar. 31, 2016
Premiums written
Direct	$ 73,408	$ 43,176
Assumed	848,682	818,957
Ceded	(377,954)	(350,458)
Net premiums written	544,136	511,675
Premiums earned
Direct	58,168	33,140
Assumed	498,999	456,641
Ceded	(191,122)	(136,175)
Net premiums earned	366,045	353,606
Claims and claim expenses
Gross claims and claim expenses incurred	251,804	161,998
Claims and claim expenses recovered	(58,723)	(35,393)
Net claims and claim expenses incurred	$ 193,081	$ 126,605