REINSURANCE (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums written
Gross									$ 5,556,273	$ 4,970,208	$ 4,603,730
Ceded									(1,529,130)	(1,217,234)	(929,064)
Net									4,027,143	3,752,974	3,674,666
Premiums earned
Gross									5,616,234	4,762,394	4,567,953
Ceded									(1,467,474)	(1,056,769)	(881,536)
Net	$ 1,211,495	$ 1,017,131	$ 981,431	$ 938,703	$ 921,879	$ 934,415	$ 946,990	$ 902,340	4,148,760	3,705,625	3,686,417
Reinsurance receivables:
Ceded losses and loss expenses									(1,010,000)	(556,000)	$ (577,000)
Provision for unrecoverable reinsurance	$ (17,000)				$ (20,000)				$ (17,000)	$ (20,000)
AM Best A minus Or Better Rating | Gross Reinsurance Recoverable
Concentration Risk [Line Items]
Percentage of gross reinsurance recoverables collectible from reinsurers rated A- or better by A.M. Best									88.80%	96.70%