REINSURANCE (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
Premiums written
Gross									$ 6,910,065	$ 5,556,273	$ 4,970,208
Ceded									(2,251,103)	(1,529,130)	(1,217,234)
Net									4,658,962	4,027,143	3,752,974
Premiums earned
Gross									6,882,217	5,616,234	4,762,394
Ceded									(2,090,722)	(1,467,474)	(1,056,769)
Net	$ 1,214,469	$ 1,224,075	$ 1,185,548	$ 1,167,402	$ 1,211,495	$ 1,017,131	$ 981,431	$ 938,703	4,791,495	4,148,760	3,705,625
Reinsurance receivables:
Ceded losses and loss expenses									(1,565,000)	(1,010,000)	(556,000)
Provision for unrecoverable reinsurance	$ 21,000				$ 17,000				$ 21,000	$ 17,000	$ 20,000