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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Premiums written
Direct	$ 64,040	$ 53,488	$ 205,253	$ 154,205
Assumed	576,229	376,736	2,184,521	1,897,280
Ceded	(157,048)	(146,002)	(806,672)	(735,672)
Net premiums written	483,221	284,222	1,583,102	1,315,813
Premiums earned
Direct	61,908	43,097	176,433	114,173
Assumed	712,499	471,097	1,731,845	1,392,278
Ceded	(226,615)	(167,673)	(612,176)	(454,922)
Net premiums earned	547,792	346,521	1,296,102	1,051,529
Claims and claim expenses
Gross claims and claim expenses incurred	2,482,510	153,846	2,924,217	540,696
Claims and claim expenses recovered	(1,260,814)	(41,271)	(1,366,853)	(133,766)
Net claims and claim expenses incurred	$ 1,221,696	$ 112,575	$ 1,557,364	$ 406,930