Schedule IV. Supplemental Schedule of Reinsurance Premiums (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Abstract]
Gross Amounts									$ 292,219	$ 244,285	$ 157,112
Ceded to Other Companies									1,095,886	833,929	628,675
Assumed									2,779,796	2,307,219	1,874,993
Net premiums earned (Note 6)	$ 574,613	$ 531,849	$ 429,385	$ 440,282	$ 421,473	$ 547,792	$ 382,265	$ 366,045	$ 1,976,129	$ 1,717,575	$ 1,403,430
Percentage of Amount Assumed to Net									141.00%	134.00%	134.00%