Reinsurance (Effect Of Reinsurance And Retrocessional Reinsurance On Premiums Written And Earned) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reinsurance Disclosures [Abstract]
Direct premiums written	$ 1,046,833	$ 957,244	$ 1,896,317	$ 1,836,332
Direct premiums earned	914,249	865,943	1,777,235	1,733,387
Assumed premiums written	310,331	320,405	921,598	833,980
Assumed premiums earned	312,538	295,868	620,107	585,931
Ceded premiums written	(219,507)	(227,103)	(420,029)	(437,806)
Ceded premiums earned	(193,213)	(210,952)	(381,166)	(410,773)
Net premiums written	1,137,657	1,050,546	2,397,886	2,232,506
Net premiums earned	$ 1,033,574	$ 950,859	$ 2,016,176	$ 1,908,545