Schedule IV. Supplemental Schedule of Reinsurance Premiums (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
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Abstract]
Gross Amounts									$ 404,525	$ 292,219	$ 244,285
Ceded to Other Companies									1,414,383	1,095,886	833,929
Assumed									4,348,261	2,779,796	2,307,219
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
Percentage of Amount Assumed to Net									130.00%	141.00%	134.00%