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, 2018	Mar. 31, 2017
Premiums written
Direct	$ 85,167	$ 73,408
Assumed	1,074,485	848,682
Ceded	(496,608)	(377,954)
Net premiums written	663,044	544,136
Premiums earned
Direct	70,032	58,168
Assumed	605,483	498,999
Ceded	(235,233)	(191,122)
Net premiums earned	440,282	366,045
Claims and claim expenses
Gross claims and claim expenses incurred	225,742	251,804
Claims and claim expenses recovered	(54,039)	(58,723)
Net claims and claim expenses incurred	$ 171,703	$ 193,081