REINSURANCE - Summary of Premiums and Losses (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
REINSURANCE
Maximum amount beyond which net loss on any individual risk is limited through the purchase of reinsurance	$ 3,000								$ 3,000
WRITTEN
Direct									934,913	$ 848,153	$ 844,430
Reinsurance assumed									48,303	37,159	30,434
Reinsurance ceded									(160,041)	(135,458)	(133,912)
Net									823,175	749,854	740,952
EARNED
Direct									896,234	835,118	835,294
Reinsurance assumed									41,926	32,521	27,886
Reinsurance ceded									(146,794)	(129,702)	(134,572)
Net amount	$ 204,002	$ 200,815	$ 196,522	$ 190,027	$ 188,296	$ 182,025	$ 184,331	$ 183,285	791,366	737,937	728,608
LOSSES AND SETTLEMENT EXPENSES INCURRED
Direct									560,421	486,986	405,873
Reinsurance assumed									20,376	16,072	13,196
Reinsurance ceded									(152,604)	(101,474)	(69,291)
Net									$ 428,193	$ 401,584	$ 349,778