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								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Premiums written
Direct									$ 461,409	$ 337,587	$ 290,730
Assumed									4,346,341	2,972,840	2,506,810
Ceded									(1,426,257)	(1,178,525)	(926,215)
Net premiums written	$ 725,367	$ 704,130	$ 1,022,965	$ 929,031	$ 411,094	$ 453,255	$ 604,509	$ 663,044	3,381,493	2,131,902	1,871,325
Premiums earned
Direct									404,525	292,219	244,285
Assumed									4,348,261	2,779,796	2,307,219
Ceded									(1,414,383)	(1,095,886)	(833,929)
Net premiums earned (Note 7)	970,125	906,748	911,502	550,028	574,613	531,849	429,385	440,282	3,338,403	1,976,129	1,717,575
Claims and claim expenses
Gross claims and claim expenses incurred									3,221,778	2,578,536	3,420,388
Claims and claim expenses recovered									(1,124,757)	(1,458,518)	(1,558,960)
Net claims and claim expenses incurred	$ 762,093	$ 654,520	$ 453,373	$ 227,035	$ 477,638	$ 410,510	$ 60,167	$ 171,703	$ 2,097,021	$ 1,120,018	$ 1,861,428