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, 2020	Mar. 31, 2019
Premiums written
Direct	$ 147,692	$ 110,968
Assumed	1,878,029	1,453,327
Ceded	(755,913)	(635,264)
Net premiums written	1,269,808	929,031
Premiums earned
Direct	134,229	89,814
Assumed	1,149,733	765,433
Ceded	(370,864)	(305,219)
Net premiums earned	913,098	550,028
Claims and claim expenses
Gross claims and claim expenses incurred	747,715	338,119
Claims and claim expenses recovered	(176,761)	(111,084)
Net claims and claim expenses incurred	$ 570,954	$ 227,035